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Amino Acid Side Effects: More Isn’t Necessarily Better

By: by Amino Science
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Amino acids are the building blocks of protein and are therefore essential nutrients the body needs. They also offer a variety of health benefits and are generally safe for consumption—both in the foods we eat and the amino acid supplements we take. And because they’re necessary for proper functioning of the body, amino acid side effects are virtually nonexistent. However, adverse reactions have been reported with consumption of excessive levels of dietary supplements, especially when amino acids have been used individually or in an unbalanced formulation.

In this article, we’re going to discuss these side effects and how you can avoid them and get the most out of amino acid supplementation.

Why Are Amino Acids Important?

As just stated, amino acids are the building blocks of protein, and protein is necessary for proper functioning of almost every biological process. For instance, without amino acids to facilitate protein synthesis, muscle tissue couldn’t be built and muscle growth couldn’t occur. And without these building blocks of life, our brains would be unable to function, as many amino acids act as both neurotransmitters in their own right and precursors to neurotransmitters.

In other words, we wouldn’t exist without amino acids. But the body requires that the amount of available amino acids be in balance, so more is not necessarily better. Furthermore, we don’t have the ability to store excess amino acids, so when too many are consumed, they’re broken down, or catabolized.

During the process of catabolism, the toxic byproduct ammonia is produced. This is then converted to urea—a nontoxic substance that’s excreted by the kidneys. If kidney function is normal, this presents little problem for the body. However, if renal function is impaired, excess amino acids may place an additional burden on the kidneys.

Yet when taken as a balanced formula, in appropriate doses, amino acid supplements have been shown by research to be generally quite safe and free of potential side effects.

However, there are some amino acids, such as arginine and methionine, that should be used with caution in certain individuals. And branched-chain amino acids (BCAAs) can disrupt the functioning of other amino acids when not taken as part of a complete amino acid formula.

Let’s take a closer look at the amino acids most likely to cause problems.

Amino Acid Side Effects

There are actually four amino acids—and one group—that are most likely to cause side effects in sensitive individuals. These are:

  • Arginine
  • Methionine
  • BCAAs
  • Lysine
  • Cysteine


Arginine is a versatile amino acid that helps remove ammonia from the body, strengthen the immune system, and trigger the production of growth hormone and blood pressure–lowering nitric oxide. This makes it a popular supplement in the treatment of erectile dysfunction. But because it helps lower blood pressure, arginine supplements aren’t recommended for people who currently take medications for hypertension.

Side effects that may occur when taking arginine supplements include abdominal pain, nausea, diarrhea, bloating, and gout. And those with liver or kidney disease should take arginine under the direction of their health care provider, as it may increase levels of potassium in the blood.

Arginine may also decrease blood sugar levels, worsen symptoms of asthma and allergies, and trigger outbreaks in people infected with the herpes virus. In addition, the amino acid should be avoided after a heart attack, as it may increase the risk of death. Therefore, people with diabetes, asthma, allergies, cold sores, or previous heart attacks should speak with their health care provider before using arginine.


Methionine is the amino acid that acts as the initiator of protein synthesis. However, it’s also a precursor of the amino acid homocysteine and can raise levels of homocysteine in the blood—high levels of which are linked to heart disease, dementia, osteoporosis, and stroke. Therefore, people taking methionine should be sure to supplement with vitamins B6, B12, and folate, as this trio of B vitamins can help lower blood levels of homocysteine.

In addition, long-term supplementation with high doses of the amino acid should be administered under the guidance of a licensed health care professional. However, methionine doesn’t present the same potential for adverse effects when used as part of a balanced mixture of essential amino acids (EAAs), as it’s largely incorporated into body protein.

Additional potential side effects associated with methionine include nausea, vomiting, and drowsiness. Women who are pregnant should avoid the amino acid as well.


The BCAAs (isoleucine, valine, and leucine) are touted for their ability to enhance muscle mass and exercise performance, decrease muscle soreness, combat insulin resistance, and lower blood sugar levels. Of the BCAAs, the amino acid L-leucine—the main form used for supplementation—is especially known for its important role in muscle building and inhibiting muscle breakdown.

However, all EAAs are necessary for proper muscle growth to occur. Moreover, when used alone, the BCAAs can block other amino acids from getting into the brain. That’s because certain amino acids use the same transporters, and the amino acid in the highest concentration is first in line when hitching a ride into the brain.

Tryptophan and tyrosine are two of the amino acids that BCAAs compete with for transport into the brain. So, for example, when you amplify the amount of BCAAs in the body, you inhibit the transport of both tryptophan and tyrosine—and this serves to reduce the neurotransmitters they help produce, including serotonin and dopamine.

To avoid this potentially harmful effect, BCAA supplements should be consumed as part of a balanced and complete amino acid formula.

Additional side effects of BCAAs (when taken in isolation) can include loss of coordination and fatigue. It’s also recommended that people with amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s disease, avoid BCAAs, as some studies have shown a link between the excitatory effect of BCAAs and the exacerbation of symptoms of ALS.

People with a condition known as maple syrup urine disease should also avoid BCAA supplementation, as this disorder results in a deficiency in the enzymes needed to properly break down these amino acids.


Lysine is a popular immune booster and virus blocker, known especially for its role in hindering the activity of the amino acid arginine, which is necessary for the growth of the virus that causes cold sores. However, when taken in excessive amounts, lysine can cause abdominal pain, diarrhea, and gallstones. Women who are pregnant or breastfeeding and people with kidney or liver disease should also avoid taking lysine supplements.


The amino acid cysteine is available in supplement form as N-acetylcysteine (NAC)—a form of the amino acid commonly used in hospitals to treat acetaminophen overdose. NAC acts as an immediate precursor to glutathione, which is also known as the body’s master antioxidant. Moreover, NAC helps prevent liver disease by assisting in the body’s natural detoxification process.

While NAC is generally well tolerated, large doses may cause headaches, nausea, vomiting, diarrhea, constipation, fatigue, and skin rashes. In addition, people who are currently taking blood pressure medications should exercise caution, as concomitant use of NAC may increase the effects of the medications. And the use of NAC with activated charcoal has also been shown to reduce the latter’s effectiveness.

Avoid the Side Effects, Reap the Benefits

There is a way to sidestep all the unwanted side effects of getting too much of an amino acid—taking a full host of amino acids as part of an essential amino acid blend. That keeps the appropriate concentration of amino acids balanced in the body. You can learn more about balanced blends of amino acids and how to properly supplement here.

Possible side effects of Amino Acids

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