Table of Contents
Key Takeaways
- SAM-e (S-Adenosyl Methionine) is a naturally occurring amino acid crucial for brain health and bodily functions.
- It regulates neurotransmitters, impacting mood, memory, and behavior.
- SAM-e treats depression and anxiety effectively, faster than traditional antidepressants and without their side effects.
- SAM-e enhances neuroplasticity, brain-cell membrane fluidity, and muscarinic receptors, leading to improved learning, memory, and mood.
- SAM-e supplementation with B vitamins at recommended dosages of 400 to 1600 mg daily benefits conditions like depression, anxiety, osteoarthritis pain, fibromyalgia, and chronic liver disease.
SAM-e (S-Adenosyl Methionine, Ademethionine, Adomet) is the naturally-occurring amino acid methionine bound to an ATP molecule. And is found in nearly every cell in your body.
SAM-e helps produce and breakdown the neurotransmitters acetylcholine, dopamine, serotonin, norepinephrine and melatonin in your brain. SAM-e maintains cell membranes and plays a role in a healthy immune system.
Studies show that SAM-e is very effective in treating depression without the side effects of prescription antidepressants. And while pharmaceutical antidepressants can take from 6 to 8 weeks to begin working, SAM-e can work much faster.
The latest research shows that SAM-e can be anti-anxiety, reduces pain including in fibromyalgia, and can improve learning, memory and mood.
SAM-e helps:
- Neuroplasticity. SAM-e is involved in the formation of myelin that surrounds and protects axons. And SAM-e can improve brain-cell membrane fluidity. Enhancing the function of neuroreceptors.
- Neurotransmitters. SAM-e is involved in the synthesis of acetylcholine, dopamine, serotonin, and norepinephrine. Helping to normalize mood, moderate behavior, and elevate emotions.
- Neuroprotectant. SAM-e helps produce the powerful antioxidant glutathione through a process called transsulfuration. SAM-e helps stabilize cell membranes and promotes the secretion of bile. And through a process called aminopropylation, SAM-e is converted into the antioxidant methylthioadenosine, which has anti-inflammatory and analgesic (pain-relieving) properties.
Overview
SAM-e (S-Adenosyl Methionine) is a naturally occurring coenzyme that plays many critical roles in your body.
SAM-e is made from methionine and ATP (adenosine triphosphate) during a cycle that recycles the amino acid homocysteine. This cycle requires Vitamin B6 and B12 and folate (B9) to work properly.
SAM-e is a precursor to the crucial antioxidant glutathione which is used in your brain and liver. When glutathione levels drop in your body, liver damage from oxidative stress begins within seconds of exposure to alcohol or toxins.
SAM-e is a methyl donor that contributes to several essential processes in your brain. As a methyl donor, SAM-e is involved in the production and recycling of hormones, cytokines, and the neurotransmitters acetylcholine, dopamine, norepinephrine, and serotonin.
This methyl donor process is call ‘methylation’, and contributes to gene expression. Affecting how your cells work and communicate including your cell DNA.
SAM-e modifies important molecules in cell membranes that control communications within and between brain cells. SAM-e boosts the number of muscarinic receptors in certain parts of your brain which are critical to this cellular communications network.
SAM-e is used to treat depression, anxiety, osteoarthritis pain, fibromyalgia, and chronic liver disease.
SAM-e is sold as an OTC dietary supplement in the United States and Canada. And as a prescription drug in several European Union countries, and Russia. SAM-e is marketed under the brand names Adomet, Gumbaral, Samyr, Heptral, Agotan, Donamet, Isimet and Admethionine.
How does SAM-e work in the Brain?
SAM-e boosts brain health and function in several ways. But two in particular stand out.
- SAM-e helps alleviate depression. SAME-e is one of the main building blocks your brain needs to produce the neurotransmitters acetylcholine, dopamine, serotonin, norepinephrine and melatonin. By raising the levels of dopamine in your brain, SAM-e helps enhance memory, motivation and learning.
Researchers at the West Los Angeles VA Medical Center studied the antidepressant effect of oral SAM-e in a randomized, double-blind, placebo-controlled trial for 15 patients with major depression.
The researchers found that SAM-e is a safe, effective antidepressant with few side effects and a rapid onset of action. And may be useful for those who cannot tolerate prescription tricyclic antidepressants.[i]
- SAM-e directly influences neuronal signaling. SAM-e increases the number of muscarinic receptors in the hippocampus. We have two kinds of acetylcholine (ACh) receptors in our brain. 1) Nicotinic receptors and 2) Muscarinic receptors
Most of the nootropics we investigate here at Nootropics Expert influence nicotinic receptors and ACh. Muscarinic receptors have a very different mechanism of action. They are part of a large family of G-protein-coupled receptors (GPCRs) which are used as an intracellular secondary messenger system.
Your brain has a very complex system of control to regulate different processes going on in different cells at different times. For this to work, there must be a sophisticated means of communication between cells.
GPCRs and their G proteins provide this intercellular communication. And form one of the most important signaling systems in your brain. They are involved in nearly every aspect of your physiology and behavior.
G proteins work by binding neurotransmitters, hormones, growth factors, cytokines, odorants and photons at the cell surface to the GPCR, and activating that receptor. Everything you see, hear, smell, or taste goes through this signaling process.
And SAM-e increases the number of these muscarinic receptors in parts of your brain, including your hippocampus. In one study, aged rats were given SAM-e for 30 days. Supplementation with SAM-e restored the number of muscarinic receptors to levels found in the same areas in young animals.[ii]
Supplementing with SAM-e to increase muscarinic receptors in your brain can boost neuroplasticity and increase learning, memory, mood and even smell and vision.
How things go bad
You have healthy levels of SAM-e throughout your body when you’re young. But as you age, your body makes less of it. This is why young people bounce back from difficult experiences more easily. They’ve got higher levels of dopamine and a higher pain threshold than adults.
SAM-e is a major methyl donor in your body. It is involved in the biosynthesis of hormones, neurotransmitters, proteins and phospholipids.[iii]
SAM-e participates in a sequence of events involving folic acid (folate) and Vitamin B12. Folate converts to 5-MTHF (5-methyltetrahydrofolate) which converts freely circulating homocysteine back into the amino acid Methionine (using Vitamin B12). L-Methionine then binds to an Adenosine group from ATP to create SAM-e.
SAM-e is then able to donate methyl groups (called methylation) to a variety of reactions including the production and breakdown of the neurotransmitters dopamine, serotonin, norepinephrine and melatonin in your brain.
This methylation process degrades SAM-e into S-Adenosylhomocysteine. Which is then fed back into this cycle from the beginning. This process is referred to as a ‘one-carbon cycle’.
If you don’t have enough folate or Vitamin B12 available, this SAM-e methylation process breaks down. And the result can be depression, brain fog, poor recall and memory, and pain.
This lack of folate and Vitamin B12 can affect you regardless of age. This is a big enough problem that the Canadian government mandated folate fortification of all flour, and some corn and rice products to address this issue in 1998.[iv]
Low levels of SAM-e, methylfolate, Vitamin B6 and B12 can lead to all kinds of problems. And genetic defects that don’t allow the use of these important vitamins can result in the same symptoms.
↓ Cognition, memory, recall, and mood diminish
↓ Methylfolate, Vitamin B6 & B12 absorption declines
↑ Homocysteine levels rise
↑ Pain levels rise
↓ Mental health, language and fine motor skills decline
All of these changes in brain energy metabolism are contributing factors to neurodegenerative diseases, including Alzheimer’s, Parkinson’s, ALS, epilepsy, dementia, and other cognitive symptoms.
But even if you’re not concerned with genetic defects, a lack of B-Vitamins, or the effects of aging, SAM-e can help.
SAM-e benefits
Stress-related disorders like anxiety, major depression and PTSD are some of the most debilitating illnesses known to man. And if you’re reading this, and dealing with any of these, supplementing with SAM-e may help.
To cope with stress requires changes in the expression of “immediate-early genes” in your hippocampus. The same area of your brain you use for learning and memory.
Stressful events result in epigenetic (gene) modifications within ‘immediate-early genes’ in your hippocampus neurons. DNA methylation acts to suppress the expression of these genes. This is where SAM-e comes in…
SAM-e is a methyl donor for the enzyme that methylates your DNA. When SAM-e levels are high enough, a stressful event will not result in DNA de-methylation.
Instead, a stressful event enhances DNA methylation of ‘immediate-early genes’. Which suppress their expression and allows you to adapt in a healthy way to this stressful situation.[v]
SAM-e is a powerful antidepressant. In 2005, researchers combed through the databases of Medline, Psychinfo, AMED, and Cochrane Controlled Trials Register. And collated the findings of randomized, controlled trials studying SAM-e for depression through to September 2001.
The team concluded after analyzing all the clinical evidence that SAM-e was effective in treating major depression in adults.[vi]
SAM-e is also a potent pain-killer. A recent study looked at 56 people with arthritis in their knees for 16 weeks. One group took the COX-2 inhibitor Celebrex while the other group took SAM-e.
Researchers found that SAM-e was as effective as Celebrex at eliminating pain. And without the potential side effects of heart attack or stroke that can be caused by using Celebrex.[vii]
Another study on the effect of SAM-e with 17 fibromyalgia patients confirmed a close relationship between primary fibromyalgia and psychologic problems including depression.
SAM-e treatment improved the depressive state of these patients. And SAM-e was found to be an effective and safe therapy in the management of fibromyalgia.[viii]
How does SAM-e feel?
For some, supplementing with SAM-e can be life-changing.
SAM-e can have a profound effect on emotions, depression, and feelings of anxiety. You may experience improved concentration, energy, alertness, and feelings of well-being. Even vision can become clearer.
Once you start supplementing with SAM-e, you should experience at the very least, a general sense of well-being.
SAM-e works particularly well for those who deal with depression or anxiety. Neurohackers report that they no longer have panic attacks. And the feeling of doom is gone.
Adding SAM-e to your nootropic stack can improve sociability, and not feeling overwhelmed by life. Brain fog lifts and thinking is clearer and faster.
Others say that the “dark cloud has lifted”, and all the negativity and stress is gone. Things that normally would irritate are just brushed off, and you move on.
One big word of caution: SAM-e needs Vitamins B6 & B12 and folate to work. Or supplementing with SAM-e may be a waste of time and money because without adequate levels of these B-Vitamins it will not provide any benefit.
You’ll notice reference to B-Vitamins several times in this article on SAM-e. It is that important. Use Vitamin B6 & B12 and folate every day that you supplement with SAM-e. Or use a B-Complex formula that contains methylfolate (B9) (avoid folic acid).
SAM-e Clinical Research
SAM-e Lifts Mood
We have made significant progress in the last 100 years in diagnosing and treating depression. And yet, 10’s of millions still suffer from depression. In 2014, an estimated 15.7 million adults in the United States had at least one major depressive episode in the past year.[ix]
In the United States, one in ten Americans are using antidepressants.[x] I don’t have statistics from other countries but I suspect many have similar problems. Unfortunately, antidepressants only work 30 – 50% of the time. And often come with a host of side effects.
One reason that many people continue to suffer from depression is that most doctors are not aware of the link between homocysteine and depression.
If you’re dealing with depression and have had little success with antidepressants, you may have something as simple (and as serious) as a folate deficiency. Or low levels of Vitamin B12. Studies show a link between folate deficiency and impaired metabolism of serotonin, dopamine, and norepinephrine.
One study looked at 46 patients with severe depression. 24 of these patients had raised levels of homocysteine. And significantly lower levels of folate, SAM-e, and other metabolites.
The researchers concluded that looking at total homocysteine levels could be a measure of depression. When it is caused by a folate deficiency, impaired methylation (SAM-e), and poor neurotransmitter metabolism (SAM-e). And a potential benefit to simply using vitamin replacement to treat severe depression.[xi]
Or supplementing with SAM-e to treat depressive symptoms.
SAM-e for ADHD in Adults
Ritalin and Adderall are thought to be the most effective treatment in children and adults with ADHD. These stimulants work by potentiating both dopamine and norepinephrine at the synaptic cleft. But stimulant meds for ADHD come with side effects.
SAM-e acts as a methyl donor and is involved in many metabolic pathways. It has both adrenergic and dopamine receptor agonist activity.
A research team at the University of California used SAM-e with subjects diagnosed with Adult ADHD in a 9-week double-blind, placebo-controlled trial.
The research team found that 75% of the patients had a significant improvement in ADHD symptoms while using SAM-e. And the 25% who did not respond to SAM-e, did not respond to Ritalin either.[xii]
SAM-e Treats Depression
Scientists at the US Department of Health and Human Services conducted an analysis of 102 individual studies in 25 databases on SAM-e and depression in 2002. The report distilled data gleaned from published studies conducted around the world up to 2002.
This comprehensive report is called, “S-Adenosyl-L-Methionine for Treatment of Depression, Osteoarthritis, and Liver Disease.” The researchers found that SAM-e is just as effective as standard antidepressant drugs at treating depression.
The agency concluded, “Treatment with SAM-e was equivalent to standard therapy for depression”.[xiii]
SAM-e Recommended Dosage
Recommended dosage of SAM-e for nootropic benefit is 400 mg per day.
SAM-e for depression: 400 – 1600 mg daily in divided doses
SAM-e for bone and joint health: 200 – 1200 mg daily in divided doses
SAM-e for liver function issues: 1600 mg daily in divided doses
SAM-e for alcoholic liver disease: 600 – 1200 mg daily in divided doses
Some research and many user reports suggest that once positive effects are achieved, SAM-e doses can be reduced. Some report benefit with as little as 100 mg of SAM-e daily.
Studies show that SAM-e should be consumed with B-Vitamins. When SAM-e donates its methyl group, it breaks down into homocysteine. Elevated homocysteine levels are associated with increased heart disease, birth defects and depression.
Choose a bioactive B-Complex to use with SAM-e. Like the high quality B-Complex like Performance Lab® B-Complex which contains therapeutic amounts of each of the B-Vitamins you need while using SAM-e.
In order to prevent homocysteine accumulating, logic tells us based on its mechanism of action that sufficient levels of B-Vitamins must be present to convert homocysteine into the potent antioxidant glutathione.
Take SAM-e on an empty stomach and an hour before eating any food. SAM-e is best digested in your intestines and not your stomach. Not letting SAM-e settle into your digestive tract could cause stomach upset.
SAM-e Side Effects
SAM-e is produced naturally in your body. So is considered well-tolerated and safe.
If you have bipolar disorder, you could develop mania when supplementing with SAM-e. So check with your doctor before using SAM-e.
If you’re on antidepressant medication, you should check with your doctor before supplementing with SAM-e.
High doses of SAM-e can cause gas, upset stomach, diarrhea, constipation, dry mouth, headache, dizziness, anxiety or skin rashes. SAM-e may also trigger an allergic reaction in some people.
Type of SAM-e to buy
SAM-e is available in 200 and 400 mg tablets.
For optimal effects with SAM-e, stable, enteric-coated tablets are recommended. SAM-e should be taken on an empty stomach, either one hour before or two hours after meals.
SAM-e is highly unstable so check expiration dates. And you should get tablets that are packed in sealed, gel-packs for freshness.
Avoid SAM-e in powder form as you’ll likely be unsatisfied with the results.
Nootropics Expert Recommendation
SAM-e 400 mg per day
I recommend using SAM-e as a nootropic supplement.
Your body does make some SAM-e on its own. But SAM-e levels decrease as we age. And you cannot get SAM-e from food.
SAM-e is critical for the methylation process needed for making important neurotransmitters including acetylcholine, dopamine, serotonin, norepinephrine and melatonin.
SAM-e is also involved in the cycle which produces the critical antioxidant glutathione. And it’s involved in maintaining brain cell membrane integrity and fluidity.
SAM-e increases muscarinic receptors in your hippocampus which boosts the activity of acetylcholine and other critical neurotransmitters in your brain. Leading to improved learning, memory and mood.
SAM-e is especially helpful if you’re dealing with depression. Or having difficulty coping with any kind of stress.
We suggest a dose of 400 mg daily. It may take a while for SAM-e to build up in your system. So be patient. Once you’re achieving the affects you want from SAM-e, you can eventually try scaling back your dose to 100 or 200 mg.
SAM-e needs Vitamins B6 & B12 and folate to work. Or SAM-e may not provide the benefit you are looking for. Most integrative medicine doctors and naturopaths recommend stacking a high quality B-Complex when using SAM-e. So make sure you stack SAM-e with the B-Vitamins listed above, or a good Vitamin B Complex formula that uses methylfolate (NOT folic acid).
A high quality B-Complex containing these bioactive B-Vitamins like the Performance Lab® B-Complex .
Join The Discussion - 392 comments
Laura
January 6, 2021
Can I take Sam-E alongside tyrosine for my anxiety and depression?
David Tomen
January 8, 2021
Laura, yes you can take the two together as long as you support SAM-e’s use with a high quality B-Complex supplement.
Mark Painter
December 17, 2020
David, would Rhodiola pose any dangerous interactions with SAM-e? If not, do you think there’d potentially be any synergistic effect if they were stacked? Thanks very much.
David Tomen
December 17, 2020
Mark, it shouldn’t be a problem. Rhodiola Rosea acts like an MAOI. Which increases dopamine levels. And SAM-e is involved in the production and breakdown of dopamine. So there may be some synergy there. But don’t forget the B-Vitamins if you’re going to use SAM-e.
Mark Painter
December 18, 2020
David, that was my speculation as well, but the affirmation of my thinking is most appreciated.
Huss
December 7, 2020
Hi,
Is SAM-e compatible with Bacopa Monnieri?
In your other article it’s mentioned:
“Bacopa is a powerful adaptogen. Which means it helps increase the effect of certain hormones when activity is low. And will block excess stimulation when activity is high.”
I wonder if this mechanism interferes with how SAM-e works.
Thoughts?
David Tomen
December 7, 2020
Huss, that’s not how adaptogens work. They’re called “adaptogens” because these supplements bring things back into balance. They don’t “block” your body’s use of natural compounds including SAM-e.
Lea
November 22, 2020
Hi David,
I read your article about SAM-e, really interesting !!
Since childhood I had problems with lack of dopamine and serotonin giving symptoms like sort of depression and lack of attention/memory recall (I did a test and I’m homozyous MTHFR A1298C). I started to take 200 mg of SAM-e and quickly I felt so well, it was fantastic but I noticed it raises my blood pressure a lot unfortunately. I don’t understand why, as my homocysteine levels are 6-7 and I don’t need to take methylated B12/B9/B6 . Why does SAM-e do this effect ?
David Tomen
November 23, 2020
Lea, it’s a good question and it shouldn’t. The only thing I can suggest is to try using a high quality B-Complex like the Life Extension supplement (https://amzn.to/3nPMOwV).
Because SAM-e donates a methyl group and uses B6, B9 & B12. If any one of those B-Vitamins is not adequate it could cause problems. In you it may be manifesting as high blood pressure.
The B-Complex I suggested is the right form and dose and will not hurt you. It’s worth a try.
Lea
November 24, 2020
Many thanks for your time and answer David
P.S.: I wanted to get “Secrets of the Optimized Brain”, I subscribed but it didn’t work (I tried 2 different email), I didn’t receive anything yet so I think there is a problem, thanks to check
David Tomen
November 25, 2020
Lea, look in your other folders like Spam. And other folders that you normally don’t check. It’s in there somewhere. If you still can’t find the email let me know and I’ll personally email you a copy.
Damian
November 8, 2020
Hello, I wanted your help to know which of the amino acids that I detail will give me better results to sleep and stay asleep without waking up.
I already tried with gaba supplements and it did not work for me I suppose it is because it cannot cross the blood-brain barrier?
Now I am among these three amino acids that I ask for your help to be able to decide.
I would like to know if the L Theanine, or the L Glycine or the L Tauryne, which of the three will give me the best result? Thank you very much for your generosity, helping us all to learn
David Tomen
November 9, 2020
Damian, it depends on your own biology but I’d try L-Theanine, L-Tryptophan and possibly L-Glycine.
Damian
November 11, 2020
David, could you please talk about the benefits of consuming Glycin as a supplement? I have seen that you have never dedicated an investigation to them as in other substances
David Tomen
November 13, 2020
Damian, L-Glycine is on my list of thing to write. Just haven’t got to it yet. But thanks for the reminder!
Tamara Neilson
December 23, 2020
Agree with this for me..I have a double slow MAOA, yet, i’m still low serotonin (in all irony), b/c i have multiple genes that indicate less than optimal levels with serotonin (receptors, etc) ..and I will agree that my genetic report was spot on. I’m higher dopamine to serotonin with a slow comt. (it explained my panic, anxiety, etc) ..I never knew that high dopamine levels as not balanced with the inhibitory could do that. I’m always “on”…on the plus side, I have incredible focus. So there’s that. The downside is the over stimulation & having a hard time with winding down. I did start to take l methyfolate + a solid B complex that has active P5P for B6 and extra sublingual b12 (mixed B forms) ..these things got my homocysteine down to a 7 (was close to 15 prior) ..(the B vitamins are truly life and crucial for DNA methylation)..I take Sam e on occasion and it really helps me to relax & l tryphophan was a downright savior when it comes to sleep and staying asleep. l theanine helps immensely too…it calms my brain down. i just feel relaxed and 5mg lithium orotate is more helpful than i would have imagined too. (and i love bacopa)…I just feel so relaxed now, yet clear. It took some experimenting, but i found out what works for me. I found your videos & they are so informative. Thank you for what you do!
David Tomen
December 24, 2020
Tamara, that is the best news I could hear. And thank you for that. Experimenting and finding what works for you with a unique system is key to making nootropics work. Patience helps too! 🙂
Shivam
September 22, 2020
Hello David, Recently I started taking sam-e 200 after finding on internet that it can solve my headache and depression problem. I want to know more from you that can it cure me of the headache and depression. I want to lead a headache free life. Also I am taking antidepressants- paxidep 25 and ventab 25 too. Beacuse after two days my headache has increased. Please tell.
David Tomen
September 23, 2020
Shivam, if you get an even stronger headache after starting SAM-e that’s a good sign that SAM-e is not your problem. And will not likely solve your headache and depression issues.
SAM-e is a naturally occuring amino acid in every cell in your body. It’s likely that your SAM-e levels are fine. And your problems are caused by something else.
Are your headaches actual “headaches” or is it a migraine?
Depression can be caused by a host of things. This post will help you understand what I mean by that. And what you can try through the process of elimination to get at the root cause of your depression: https://nootropicsexpert.com/best-nootropics-for-depression/
Carl
September 15, 2020
Hi David,
Sam E works well to control my anxiety. However, it either upset my stomach badly if I take it right before meal. Or if I take it with empty stomach with meals at least 2 hours afterwards, it hurt my intestine badly. Is there any brand of Sam E that doesn’t hurt digestion system that much?
I read that it may also be taken via injection. Do you recommend this? Where can I get this done?
Thanks!
Carl
David Tomen
September 15, 2020
Carl, are they enteric coated capsules or tablets? And what size of dose is it?
Some alternative or complementary medicine doctors and clinics offer SAM-e by IV. You’ll need to check around your area.
Carl
September 16, 2020
Hi David,
I use enteric coated tablets 400mg. I do have sensitive digestive system….
David Tomen
September 16, 2020
Carl, they just be enteric coated capsules and sealed in blister packs because this compound is so unstable.
If SAM-e works for you then I suggest investing in some trial and error and trying a couple of other brands. The only brand I was able to find that had zero “other ingredients” were in capsules in a bottle rather than blister packs. And I don’t recommend them because there is no way they can maintain their potency.
Doctor’s Best SAM-e is only 200 mg and might be worth trying: https://amzn.to/2FsgXlG.
Jarrow Formulas SAM-e is 400 mg and also worth checking out: https://amzn.to/2HbA7wF
If you have a sensitive digestive system you need to find the right “form” of SAM-e and find a supplement with the least amount of “other ingredients” as you can.
Sherry
September 9, 2020
I have been taking Sam-e for 60 days. It has been a life changing experience. I have read through the comments and wanted to share that I also, like one other person said, developed a huge headache in the beginning. I found on a different forum where a person experiencing headaches took a lower dose more often during the day and the headache went away. It worked for me. I have worked my dose up to 800 mg a day. If I miss my afternoon dose, I get overwhelmingly tired and the headache is mildly there. I plan on increasing my dosage slowly. I take B vitamins. I wanted to share that my libido has increased tremendously! First time in 25 years I have felt so good. Will this last? On days when I am struggling with the fatigue I add ginkgo. Anything else you can direct me towards? In the past I tried Wellbutrin with minimum results. I tried other antidepressants, they all terribly upset the GI and make me sleep nonstop.
David Tomen
September 10, 2020
Sherry, thank you for sharing your experience with SAM-e. It’s comments like yours that are a tremendous help to our community.
Ginkgo Biloba acts like an MAOI which boosts dopamine and norepinephrine. If you find it helps that provides a hint that you may be low in dopamine. You can try L-Tyrosine which is a precursor to the production of dopamine.