Carbidopa in combination with levodopa is used in treating Parkinson's disease. This disease is associated with a deficit of dopamine, a neurotransmitter (a chemical messenger in the brain). By preventing the breakdown of levodopa in the general circulation, carbidopa enables more levodopa to enter the brain, where it is converted into dopamine.
The body uses the natural substance 5-hydroxytryptophan (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. 1,2,3 Because of the risk of this side effect, if you take levodopa/carbidopa for Parkinson's disease, avoid supplemental 5-HTP.
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. 4 Because it is the amino acids in proteins that affect levodopa, BCAAs might cause the same problem. Therefore, if you take levodopa/carbidopa for Parkinson's, it may be advisable to avoid BCAAs and other amino acid supplements.
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% and 75%, respectively, by iron supplementation, resulting in a worsening of symptoms of Parkinson's disease. 5 Based on this finding, you should separate the times you take iron and these drugs by as long as possible.
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. 6 This could at least partially neutralize the therapeutic effects of levodopa. In one individual, parkinsonism symptoms got worse following supplementation with kava extract (150 mg twice daily for 10 days). 7
Based on these reports, it may be advisable to avoid kava during levodopa/carbidopa therapy.
Certain herbal formulas used in Traditional Chinese Herbal Medicine to treat upset stomach might reduce the effectiveness of levodopa. 14
If you are taking levodopa alone, you should not take more than 5 mg per day of vitamin B 6 or it might impair the effectiveness of the drug. 13 But if you use levodopa/carbidopa combinations that provide a total daily dose of at least 75 mg of carbidopa, this issue is not a concern.
S-adenosylmethionine 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. 8,9 This suggests (but definitely does not prove) that individuals taking levodopa/carbidopa might benefit from SAMe supplements.
One short-term (30-day) double-blind study suggests that such combination treatment is safe and might help depression related to Parkinson's disease. 10 However, there are also concerns that SAMe could cause levodopa to be less effective over time. 11
The bottom line: If you are taking levodopa/carbidopa, consult your physician about whether you should take SAMe as well.
References
REF1 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;303:782–787.
REF2 Joly P, Lampert A, Thomine E, et al. Development of pseudobullous morphea and scleroderma-like illness during therapy with L-5-hydroxytryptophan and carbidopa. J Am Acad Dermatol. 1991;25:332–333.
REF3 Auffranc JC, Berbis P, Fabre JF, et al. Sclerodermiform and poikilodermal syndrome observed during treatment with carbidopa and 5-hydroxytryptophan [translated from French]. Ann Dermatol Venereol. 1985;112:691–692.
REF4 Robertson DR, Higginson I, Macklin BS, et al. The influence of protein containing meals on the pharmacokinetics of levodopa in healthy volunteers. Br J Clin Pharmacol . 1991;31:413–417.
REF5 Campbell NR, Hasinoff BB. Iron supplements: a common cause of drug interactions. Br J Clin Pharmacol . 1991;31:251–255.
REF6 Schelosky L, Raffauf C, Jendroska K, et al. Kava and dopamine antagonism [letter]. J Neurol Neurosurg Psychiatry. 1995;58:639–640.
REF7 Schelosky L, Raffauf C, Jendroska K, et al. Kava and dopamine antagonism [letter]. J Neurol Neurosurg Psychiatry. 1995;58:639–640.
REF8 Liu X, Lamango N, Charlton C. L-dopa depletes S-adenosylmethionine and increases S-adenosyl homocysteine: Relationship to the wearing off effects. Soc Neurosci. 1998;24:1469.
REF9 Bottiglieri T, Hyland K, Reynolds EH. The clinical potential in ademetionine (S-adenosylmethionine) in neurological disorders. Drugs. 1994;48:137–152.
REF10 Carrieri PB, Indaco A, Gentile S, et al. S-adenosylmethionine treatment of depression in patients with Parkinson's disease: a double-blind, crossover study versus placebo. Curr Ther Res. 1990;48:154–160.
REF11 Liu X, Lamango N, Charlton C. L-dopa depletes S-adenosylmethionine and increases S-adenosyl homocysteine: Relationship to the wearing off effects. Soc Neurosci. 1998;24:1469.
REF12 Snider SR. Octacosanol in parkinsonism [letter]. Ann Neurol. 1984;16:723.
REF13 Drug evaluations subscription (section 4, chapter 2). Volume I. Chicago: American Medical Association, Summer 1992: 12.
REF14 Sunagane N, Aikawa M, Ohta T, 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 . 2006;126:1191-6.