The body uses the natural substance 5-hydroxytryptophan (5-HTP) to manufacture serotonin, and supplemental forms of 5-HTP have been used for treating depression and migraine headaches. Since it is converted by the body to serotonin, 5-HTP might have antidepressant properties. For this reason, some people with Parkinson's-related depression have tried it.

However, the combination of 5-HTP and carbidopa might cause a scleroderma-like condition, in which the skin becomes hard and tight.^1,2,3 Because of the risk of this side effect, if you take levodopa/carbidopa for Parkinson's disease, avoid supplemental 5-HTP.

Branched-chain amino acids (BCAAs) in supplement form have been used to improve appetite in cancer patients and to slow the progression of amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease).

Dietary protein can decrease the effectiveness of levodopa in Parkinson's disease.⁴ Because it is the amino acids in proteins that affect levodopa, BCAAs might cause the same problem. Therefore, if you take levodopa/carbidopa for Parkinson's, it may be advisable to avoid BCAAs and other amino acid supplements.

Iron appears to interfere with the absorption of both levodopa and carbidopa by binding to them. Studies have found that blood levels of levodopa and carbidopa are reduced 30 to 51% and 75%, respectively, by iron supplementation, resulting in a worsening of symptoms of Parkinson's disease.⁵ Based on this finding, you should separate the times you take iron and these drugs by as long as possible.

The herb kava (Piper methysticum) has a sedative effect and is used for anxiety and insomnia.

A few case reports suggest that kava might interfere with the action of dopamine in the body.⁶ This could at least partially neutralize the therapeutic effects of levodopa. In one individual, parkinsonism symptoms got worse following supplementation with kava extract (150 mg twice daily for 10 days).⁷

Based on these reports, it may be advisable to avoid kava during levodopa/carbidopa therapy.

Certain herbal formulas used in Traditional Chinese Herbal Medicine to treat upset stomach might reduce the effectiveness of levodopa.¹⁴

If you are taking levodopa alone, you should not take more than 5 mg per day of vitamin B ₆ or it might impair the effectiveness of the drug.¹³ But if you use levodopa/carbidopa combinations that provide a total daily dose of at least 75 mg of carbidopa, this issue is not a concern.

Policosanol may increase both the effects and side effects of levodopa.¹²

S-adenosylmethionine is a naturally occurring compound derived from the amino acid methionine and the energy molecule adenosine triphosphate (ATP). SAMe is widely used as a supplement for treatment of osteoarthritis and depression.

Preliminary evidence suggests that levodopa might deplete levels of SAMe in the body.^8,9 This suggests (but definitely does not prove) that individuals taking levodopa/carbidopa might benefit from SAMe supplements.

One short-term (30-day) double-blind study suggests that such combination treatment is safe and might help depression related to Parkinson's disease.¹⁰ However, there are also concerns that SAMe could cause levodopa to be less effective over time.¹¹

The bottom line: If you are taking levodopa/carbidopa, consult your physician about whether you should take SAMe as well.