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
| 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.
| May Be Beneficial: Supportive
interaction—Taking these supplements may support or otherwise help your medication
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.
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
An asterisk (*) next to an item in the summary indicates that the
interaction is supported only by weak, fragmentary, and/or contradictory scientific
Interactions with Dietary Supplements
Iron supplements taken with carbidopa may interfere with the action of the
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
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.
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.
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.
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
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
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
12. Bender DA, Earl CJ, Lees AJ. Niacin depletion in Parkinsonian
patients treated with L-dopa, benserazide and carbidopa. Clin Sci (Colch)
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].