A study in animals has found that carbidopa inhibits an enzyme involved in the synthesis of niacin in the body.1 In addition, there is evidence that niacin synthesis is decreased in people taking carbidopa and other drugs in its class,2 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.3 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.4
5-HTP and carbidopa have been reported to improve intention myoclonus (a neuromuscular disorder) in some human cases but not others.5 , 6 , 7 Several cases of scleroderma-like illness have been reported in patients using carbidopa and 5-HTP for intention myoclonus.8 , 9 , 10
Test tube,11 animal,12 and preliminary human studies13 suggest that carbidopa may cause depletion of vitamin B6. However, the use of carbidopa with levodopa reduces the vitamin B6-depleting effects of levodopa.14 More research is needed to determine whether vitamin B6 supplementation is advisable when taking carbidopa.
1. Bender DA, Smith WR. Inhibition of kynurenine hydrolase by benserazide, carbidopa and other aromatic hydrazine derivatives: evidence for sub-clinical iatrogenic niacin deficiency. Biochem Soc Trans 1978;6:120–2.
2. Bender DA, Earl CJ, Lees AJ. Niacin depletion in Parkinsonian patients treated with L-dopa, benserazide and carbidopa. Clin Sci 1979;56:89–93.
3. Linazasoro G, Gorospe A. [Treatment of complicated Parkinson disease with a solution of levodopa- carbidopa and ascorbic acid]. Neurologia 1995;10:220–3 [Article in Spanish].
4. Campbell NR, Hasinoff BB. Iron supplements: A common cause of drug interactions. Br J Clin Pharmacol 1991;31:251–5 [review].
5. Van Woert MH, Rosenbaum D, Howieson J, Bowers MB Jr. Long-term therapy of myoclonus and other neurologic disorders with L-5-hydroxytryptophan and carbidopa. N Engl J Med 1977;296:70–5.
6. Magnussen I, Dupont E, Engbaek F, de Fine Olivarius B. Post-hypoxic intention myoclonus treated with 5-hydroxytryptophan and an extracerebral decarboxylase inhibitor. Acta Neurol Scand 1978;57:289–94.
7. Growdon JH, Young RR, Shahani BT. L-5-hydroxytryptophan in treatment of several different syndromes in which myoclonus is prominent. Neurology 1976;26:1135–40.
8. Sternberg EM, Van Woert MH, Young SN, et al. Development of a scleroderma-like illness during therapy with L-5-hydroxytryptophan and carbidopa. N Engl J Med 1980;303:782–7.
9. Joly P, Lampert A, Thromine E, Lauret P. Development of pseudobullous morphea and scleroderma-like illness during therapy with L-5-hydroxytryptophan and carbidopa. J Am Acad Dermatol 1991;25:332–3.
10. Auffranc JC, Berbis P, Fabre JF, et al. Sclerodermiform and poikilodermal syndrome observed during treatment with carbidopa and 5-hydroxytryptophan. Ann Dermatol Verereol 1985;112:691–2.
11. Bender DA. Inhibition in vitro of the enzymes of the oxidative pathway of tryptophan metabolism and of nicotinamide nucleotide synthesis by benserazide, carbidopa and isoniazid. Biochem Pharmacol 1980;29:707–12.
12. Bender DA. Effects of benserazide, carbidopa and isoniazid administration on tryptophan-nicotinamide nucleotide metabolism in the rat. Biochem Pharmacol 1980;29:2099–2104.
13. Bender DA, Earl CJ, Lees AJ. Niacin depletion in Parkinsonian patients treated with L-dopa, benserazide and carbidopa. Clin Sci (Colch) 1979;56:89–93.
14. Trovato A, Nuhlicek DN, Midtling JE. Drug-nutrient interactions. Am Fam Physician 1991;44:1651–8.
Last Review: 05-01-2013
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