Carbidopa

Test tube, animal, and preliminary human studies suggest that carbidopa may cause depletion of vitamin B6. However, the use of carbidopa with levodopa reduces the vitamin B6-depleting effects of levodopa. More research is needed to determine whether vitamin B6 supplementation is advisable when taking carbidopa.

A study in animals has found that carbidopa inhibits an enzyme involved in the synthesis of niacin in the body. In addition, there is evidence that niacin synthesis is decreased in people taking carbidopa and other drugs in its class, raising the concern that people taking these drugs could be at risk of niacin deficiency, even if not frankly deficient. Further studies will be required determine if niacin supplementation is appropriate in people taking carbidopa.

A combination of levodopa/carbidopa and vitamin C may be useful for people with Parkinson’s disease whose motor complications are not effectively managed with conventional drug treatment. This combination was administered to people with Parkinson’s disease for 16.8 months in an unblinded, uncontrolled study. The researchers reported that participants who completed the study experienced substantial increases in the number of hours with good functional capacity and were able to reduce their intake of other anti-Parkinsonian drugs. However, 62% of the participants withdrew from the study, citing difficulty in performing voluntary movements as the main reason. Until more research is performed, this drug-nutrient combination must be viewed as preliminary.

Iron supplements taken with carbidopa may interfere with the action of the drug.

5-HTP and carbidopa have been reported to improve intention myoclonus (a neuromuscular disorder) in some human cases but not others. Several cases of scleroderma-like illness have been reported in patients using carbidopa and 5-HTP for intention myoclonus.