Carbidopa 
Summary of Interactions with Vitamins, Herbs, and Foods
In some cases, an herb or supplement may appear in more than one category, which may seem contradictory. For clarification, read the full article for details about the summarized interactions.
 May Be Beneficial: Depletion or interference—The medication may deplete or interfere with the absorption or function of the nutrient. Taking these nutrients may help replenish them. | Niacin* |
| May Be Beneficial: Supportive interaction—Taking these supplements may support or otherwise help your medication work better. | Vitamin C* |
 Avoid: Reduced drug absorption/bioavailability—Avoid these supplements when taking this medication since the supplement may decrease the absorption and/or activity of the medication in the body. | Iron |
 Check: Other—Before taking any of these supplements or eating any of these foods with your medication, read this article in full for details. | 5-HTP, Vitamin B6 (see text) |
| Side effect reduction/prevention | None known |
| Adverse interaction | None known |
An asterisk (*) next to an item in the summary indicates that the interaction is supported only by weak, fragmentary, and/or contradictory scientific evidence.
Interactions with Dietary Supplements
Iron
Iron supplements taken with carbidopa may interfere with the action of the drug.1
5-Hydroxytryptophan (5-HTP)
5-HTP and carbidopa have been reported to improve intention myoclonus (a neuromuscular disorder) in some human cases but not others.2 3 4 Several cases of scleroderma-like illness have been reported in patients using carbidopa and 5-HTP for intention myoclonus.5 6 7
Niacin
A study in animals has found that carbidopa inhibits an enzyme involved in the synthesis of niacin in the body.8 In addition, there is evidence that niacin synthesis is decreased in people taking carbidopa and other drugs in its class,9 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.
Vitamin B6
Test tube,10 animal,11 and preliminary human studies12 suggest that carbidopa may cause depletion of vitamin B6. However, the use of carbidopa with levodopa reduces the vitamin B6-depleting effects of levodopa.13 More research is needed to determine whether vitamin B6 supplementation is advisable when taking carbidopa.
Vitamin C
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.14 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.
References (To view, roll mouse over the "References" heading; to hide, click on the heading)
1. Campbell NR, Hasinoff BB. Iron supplements: A common cause of drug interactions. Br J Clin Pharmacol 1991;31:251–5 [review].
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. Bender DA, Earl CJ, Lees AJ. Niacin depletion in Parkinsonian patients treated with L-dopa, benserazide and carbidopa. Clin Sci 1979;56:89–93.
10. 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.
11. Bender DA. Effects of benserazide, carbidopa and isoniazid administration on tryptophan-nicotinamide nucleotide metabolism in the rat. Biochem Pharmacol 1980;29:2099–2104.
12. 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.
13. Trovato A, Nuhlicek DN, Midtling JE. Drug-nutrient interactions. Am Fam Physician 1991;44:1651–8.
14. 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].
Copyright © 2008 Healthnotes, Inc. All rights reserved. www.healthnotes.com
Please read the disclaimer about the limitations of the information provided here. Do NOT rely solely on the information in this article.
Learn more about Healthnotes, the company.
Learn more about the authors of Using Medicines with Vitamins and Herbs
The information presented in Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires June 2009.