Evidence: Reasonable Possible Benefits: High Safety: Reasonable
TABLE OF CONTENTS
What is L-cysteine?
Cysteine is often called a non-essential amino acid, which means you do not need to consume it to survive. However, in rare cases of illness, the body cannot produce enough cysteine from methionine and NAC from L-cysteine, which is why both L-cysteine and NAC can be considered conditionally essential amino acids. Cysteine can be found in all whole foods with dietary protein. As a supplement, it is often used for reducing symptoms of certain diseases, reducing acne, detoxifying from heavy metals, and more. This article is about the main benefits of L-cysteine, its potential side effects, the best time to take it, and more.
What does L-cysteine do for the body?
Cysteine is often called a non-essential amino acid, which means you do not need to consume it to survive. However, in rare cases of illness, the body cannot produce enough cysteine from methionine and serine, and NAC from L-cysteine, which is why both L-cysteine and NAC can be considered conditionally essential amino acids.
In the body, L-cysteine can turn into NAC (N-Acetyl-L-Cysteine) and vice versa.
NAC assists in the production of glutathione, considered by many to be “the mother of all antioxidants.” Because of this, the amino acid is necessary for proper lung function, brain function, and liver detoxification.
The amino acid also helps regulate glutamate levels, influencing the neurons in the central nervous system.
L-cysteine is also necessary to break down mucus, keep men fertile, synthesize taurine and other molecules, and support digestive health.
Possible benefits
All of the potential benefits mentioned below are dose-dependent. Generally, the higher the dose, the more pronounced the effects are. However, they do reach a plateau at a certain point. We will discuss the best dosing protocol later in this article.
Major benefits
Reducing the symptoms of cysteine deficiency – Cysteine deficiency can lead to decreased production of peptide glutathione, leading further to edema, liver damage, muscle loss, lethargy, skin lesions, and weakness. Fortunately, this deficiency is rare and shouldn’t happen to healthy individuals who consume at least 0.36 grams of protein/pound of body weight.
Reduced craving and addiction – After about two weeks of 2,400mg NAC supplementation, cigarette usage appears to be reduced voluntarily by about 25%. This benefit has also been shown in cocaine and marijuana users and likely applies to other addictions as well.
Minor benefits
Reduced acne – NAC is one of the most effective supplements to reduce acne. This benefit usually occurs in people who take the supplement for at least 2 months.
Heavy metal detoxification – NAC has been shown to reduce lead bioaccumulation in red blood cells. This effect likely applies to other heavy metals, such as mercury and arsenic, as well.
Reduced symptoms of angina – Taking L-cysteine along with nitroglycerin, a drug that opens up blood vessels and improves blood flow, can reduce chest pain, heart attack, and the risk of death related to angina. However, the combination can also lead to severe headaches and other side effects.
Reduced symptoms of COPD – N-Acetyl-L-Cysteine can reduce some symptoms of COPD (Chronic bronchitis and chronic obstructive pulmonary disease) by acting as a mucolytic agent and reducing sputum formation. The supplement can lower the number of flare-ups by about 40%.
Reduced symptoms of trichotillomania – Preliminary evidence suggests a 31-45% reduction in hair-pulling symptoms in persons with trichotillomania when supplementing with 1,200-2,400mg NAC for twelve weeks.
Reduced symptoms of flu – In one 6-month study, people who took 600 mg of NAC two times a day had fewer flu symptoms than those who took a placebo. This effect may apply to L-cysteine as well although to a lesser degree.
Reduced symptoms of Tylenol poisoning – Intravenous NAC can help prevent or reduce liver and kidney damage related to a Tylenol (acetaminophen) overdose. This benefit likely applies to oral NAC and L-cysteine supplements as well.
Possible side effects
These side effects are dose-dependent. The risk for them increases (often linearly but sometimes exponentially) as you increase the dose. Some of the side effects only apply to very high doses.
Nausea
Diarrhea
Vomiting
Headache
Raised levels of homocysteine – While to a much lesser degree than L-methionine, both NAC and L-cysteine supplements can increase homocysteine levels to some degree, especially when taken at high doses.
Amino acid imbalance – Essential and non-essential amino acids should be kept in balance as they act synergistically in many ways. Too much of one amino acid can sometimes cause side effects if there isn’t enough of another amino acid. For example, this principle applies to glycine and methionine, lysine and arginine, and the BCAA trio. There are many other proven and unproven examples. Food or a protein powder is usually a better choice for getting enough amino acids unless you have a specific goal in mind targeted by a particular amino acid/s.
Toxicity – While acute toxicity is extremely rare, taking high amounts of L-cysteine (typically above 7 grams) can lead to chronic toxicity and cause some health problems. To prevent chronic toxicity, it is best to stay below 1 gram a day unless you have a great reason to take more.
Contamination – Contaminated supplements are uncommon in the USA and other well-regulated countries. However, if you wish to buy products from China, India, or other countries without strict regulations, beware that the supplements may be contaminated. Either way, you need to choose the brand you order from wisely. It is highly recommended to check the certificates or read through some reviews for the specific product before you buy it. The FDA (Food and Drug Administration) has the power to regulate dietary supplements but can only do so after they have been on the market for a while. That’s because supplement companies are not obligated to announce to the FDA when releasing a new product on the market. The FDA has to discover the product and test it for potential impurities, which can take a lot of time. During that time, the supplement can be sold on the market even if it’s impure. Also, supplement companies can change their manufacturers without announcing it to the FDA.
Who should not take L-cysteine?
You should probably avoid taking the supplement if you:
are willing to take NAC instead (NAC is more studied, easier to absorb, and generally safer than L-cysteine)
get enough cysteine from food
experience a severe adverse reaction after taking the amino acid
are on a tight supplement budget (since there are more cost-effective supplements on the market for most people)
Who will benefit the most?
You should consider taking the supplement if you:
consume very little protein (less than 0.3 grams/pound of body weight)
suffer from angina, COPD, trichotillomania, flu, or HIV
have high levels of heavy metals in the body
experience acute or chronic Tylenol poisoning
are addicted to nicotine, cocaine, marijuana, or other substances/habits
have acne
How much L-cysteine should you take?
The combined RDA (recommended daily allowance) for methionine and cysteine is 19 mg/kg/day for all adult men and most adult women. This is enough for the majority of people to prevent any symptoms of deficiency.
The RDA increases to 25 and 26 mg/kg/day for pregnant and lactating women, respectively.
Since these amino acids can be found in almost every food, you probably already get sufficient amounts from food alone. About 0.36 grams of protein/pound of body weight should be enough to prevent a deficiency.
If you plan to take L-cysteine as a supplement, 500 mg appears to be the best daily target for most people. This amount should provide most of the benefits without meaningful side effects.
Some people may benefit from doses higher than 500 mg. These include nicotine and cocaine addicts, people suffering from angina, COPD, trichotillomania, flu, or HIV, people with high levels of heavy metals in the body, and more.
The upper safety limit for L-Cysteine is not well-established. To stay on the safe side, don’t take more than 1 gram daily unless you have a great reason to do so. Higher amounts may cause problems if you take the supplement for a prolonged period.
Food sources of L-cysteine
L-cysteine is found in all whole foods that contain dietary protein.
The amount of cysteine per gram of protein varies from food to food but a general rule of thumb is that the more protein you eat, the more cysteine you are getting from food.
The easiest way to find out exactly how much L-cysteine or other amino acids you are getting from food is with Cronometer.com. This free app allows you to track all vitamins, minerals, and more.
Best time to take L-cysteine
Like most other amino acids, cysteine is water-soluble, so you don’t have to take it with food to absorb it well. Taking the supplement on empty stomach outside of your meal is usually better so it won’t compete with other amino acids for absorption.
Whether you take the supplement in the morning or the evening doesn’t matter much. It should not interrupt your sleep in any way.
Unless it feels too impractical, spread the daily dose into 2+ smaller doses throughout the day.
Interactions
Other amino acids Essential and non-essential amino acids should be kept in balance as they act synergistically in many ways. Too much of one amino acid can sometimes cause side effects if there isn’t enough of another amino acid. For example, this principle applies to glycine and methionine, lysine and arginine, and the BCAA trio. There are many other proven and unproven examples. Food or a protein powder is usually a better choice for getting enough amino acids unless you have a specific goal in mind targeted by a particular amino acid/s.
Alcohol NAC can minimize liver toxicity caused by alcohol to some extent.
Activated charcoal Activated charcoal might make L-cysteine supplements less effective.
Nitroglycerin While the combination can be effective in reducing chest pain and the symptoms of angina, it can also cause severe headaches and may not be safe in the long term.
Where to buy L-cysteine
Amazon is the best option for ordering L-cysteine supplements in most countries. They offer some very affordable products backed by many positive reviews. Also, you can choose from a wide range of brands without having to search through other markets on the internet.
Beware some brands display the dosage per serving instead of per pill or capsule. Therefore, you may accidentally buy something less potent than you intended. Do not fall for this marketing trick.
FAQ
You can take the amino acid daily and do not need to cycle it. However, it is certainly not a problem if you don’t take it daily. Not taking the supplement every once in a while could lead to better absorption. That is yet to be proven or disproven.
All of these options are fine. The two most important things to consider are the price and dosing. Powders are almost always the cheapest form. However, you may need a highly accurate scale to dose them correctly (preferably 0.001g). You can get one for as little as $20 from Amazon. Dosing the powders also takes some time that can add up over months or years. The disadvantage of capsules is that they often contain bovine gelatin, which rarely comes from an ethical source.
Yes. Most amino acids, including L-cysteine, absorb and digest well if you take them before falling asleep. The supplement should not impact sleep quality in any significant way.
This amino acid should not become harmful once it exceeds the expiration date but can lose potency over time.
Keep the supplement in a cold, dark, and dry place, and it should remain just as potent for many months or even years.
While toxicity is rare, too much L-cysteine in the system can cause some health problems. It is best to stay below 1 gram a day to prevent these side effects.
You may need more of this amino acid if you: – eat very little protein (less than 0.3 grams/pound of body weight) – have high levels of heavy metals in the body – suffer from angina, COPD, trichotillomania, flu, or HIV – experience acute or chronic Tylenol poisoning – are addicted to nicotine, cocaine, marijuana, or other substances/habits – have acne
The half-life of cysteine in the body is about 6 hours. This amino acid stays in your system for about 24 hours after you consume it.
In the long term, you need to take the supplement for about 2-4 weeks to begin noticing the benefits.
You can easily find vegan-friendly L-cysteine pills and powders but always check the label to be sure. If you buy the amino acid in a capsulated form, beware that the capsules often contain animal-based gelatin. Instead, find a brand that puts its products into cellulose capsules.
Like most other amino acids, L-cysteine is water-soluble, so you don’t have to take it with food to absorb it well.
L-cysteine should not break a fast unless you take a very high dose at once. If you practice time-restricted eating, take this amino acid during your eating window so it doesn’t break your fast.
References
Most of the information provided in this guide is supported by scientific research that can be found and verified in the PubMed medical library. We highly encourage you to use the library to verify anything said in this article. We excluded from consideration studies that are either confounded or have a high conflict of interest.
