What is levodopa/carbidopa? How does it interact with other drugs?

Quick Answer
Levodopa in combination with carbidopa is used in treating Parkinson’s disease.
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5-HTP (5-hydroxytryptophan)

Effect: Possible Harmful Interaction

The body uses the natural substance 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. Because of the risk of this side effect, persons taking levodopa/carbidopa for Parkinson’s disease should avoid supplemental 5-HTP.

Branched-Chain Amino Acids (BCAAs)

Effect: Possible Harmful Interaction

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. Because it is the amino acids in proteins that affect levodopa, BCAAs might cause the same problem. Therefore, if one takes levodopa/carbidopa for Parkinson’s, it may be advisable to avoid BCAAs and other amino acid supplements.


Effect: Take at a Different Time of Day

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 percent and 75 percent, respectively, by iron supplementation, resulting in a worsening of symptoms of Parkinson’s disease. Based on this finding, one should separate the times one takes iron and these drugs by as long as possible.


Effect: Possible Harmful Interaction

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. This could at least partially neutralize the therapeutic effects of levodopa. In one individual, Parkinson’s disease symptoms got worse following supplementation with kava extract of 150 milligrams (mg) twice daily for ten days. Based on these reports, it may be advisable to avoid kava during levodopa/carbidopa therapy.

Traditional Chinese Herbal Medicine

Effect: Possible Reduced Action of Drug

Certain herbal formulas used in traditional Chinese herbal medicine to treat upset stomach might reduce the effectiveness of levodopa.

Vitamin B6

Effect: Possible Reduced Action of Drug

If taking levodopa alone, one should not take more than 5 mg per day of vitamin B6 or it might impair the effectiveness of the drug. However, if one uses levodopa/carbidopa combinations that provide a total daily dose of at least 75 mg of carbidopa, this issue is not a concern.


Effect: Possible Benefits and Risks

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

SAMe (S-adenosylmethionine)

Effect: Possible Benefits and Risks

SAMe 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. This suggests (but definitely does not prove) that persons taking levodopa/carbidopa might benefit from SAMe supplements.

One short-term (thirty-day) double-blind study suggests that such combination treatment is safe and might help depression related to Parkinson’s disease. However, there are also concerns that SAMe could cause levodopa to be less effective over time. Persons taking levodopa/carbidopa should consult their physician about taking SAMe.


Bottiglieri, T., K. Hyland, and E. H. Reynolds. “The Clinical Potential in Ademetionine (S-adenosylmethionine) in Neurological Disorders.” Drugs 48 (1994): 137-152.

Liu, X., N. Lamango, and C. Charlton. “L-dopa Depletes S-adenosylmethionine and Increases S-adenosyl Homocysteine: Relationship to the Wearing Off Effects.” Social Neuroscience 24 (1998): 1469.

Sunagane, N., et al. “Possibility of Interactions Between Prescription Drugs and OTC Drugs (2nd Report): Interaction Between Levodopa Preparation and OTC Kampo Medicines for Upset Stomach.” Yakugaku Zasshi 126 (2006): 1191-1196.