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first, a reference brief on carbidopa only tablet: Generic Name Carbidopa Trade Name Lodosyn Drug Class l-aromatic acid decarboxylase inhibitor Indications/Clinical Uses coadministered with levodopa to reduce the side effects associated with levodopa therapy decreases the metabolism of levodopa in the GI tract and peripheral tissues-->increases the availability of levodopa to the CNS--> Pharmacodynamics enhances the therapeutic response--> allows for a lower dose of levodopa to be used--> decreases reduces the side effects associated with levodopa Adverse reactions/side effects Pharmacokinetics does not cross the BBB not appreciably metabolized next: Carbidopa-Levodopa Carbidopa, an inhibitor of aromatic amino acid decarboxylation, is a white, crystalline compound, slightly soluble in water, with a molecular weight of 244.3. It is designated chemically as (-)-L-alpha-hydrazino-alpha-methyl-beta- (3,4-dihydroxybenzene) propanoic acid monohydrate. Tablet content is expressed in terms of anhydrous carbidopa which has a molecular weight of 226.3. Levodopa, an aromatic amino acid, is a white, crystalline compound, slightly soluble in water, with a molecular weight of 197.2. It is designated chemically as (-)-L-alpha-amino-beta-(3,4-dihydroxybenzene) propanoic acid. ACTIONS/CLINICAL PHARMACOLOGY Current evidence indicates that symptoms of Parkinson's disease are related to depletion of dopamine in the corpus striatum. Administration of dopamine is ineffective in the treatment of Parkinson's disease apparently because it does not cross the blood-brain barrier. However, levodopa, the metabolic precursor of dopamine, does cross the blood-brain barrier, and presumably is converted to dopamine in the basal ganglia. This is thought to be the mechanism whereby levodopa relieves symptoms of Parkinson's disease. When levodopa is administered orally it is rapidly converted to dopamine in extracerebral tissues so that only a small portion of a given dose is transported unchanged to the central nervous system. For this reason, large doses of levodopa are required for adequate therapeutic effect and these may often be attended by nausea and other adverse reactions, some of which are attributable to dopamine formed in extracerebral tissues. Since levodopa competes with certain amino acids, the absorption of levodopa may be impaired in some patients on a high protein diet. Carbidopa inhibits decarboxylation of peripheral levodopa. It does not cross the blood-brain barrier and does not affect the metabolism of levodopa within the central nervous system. Since its decarboxylase inhibiting activity is limited to extracerebral tissues, administration of carbidopa with levodopa makes more levodopa available for transport to the brain. In dogs, reduced formation of dopamine in extracerebral tissues, such as the heart, provides protection against the development of dopamine-induced cardiac arrhythmias. Clinical studies tend to support the hypothesis of a similar protective effect in humans although controlled data are too limited at the present time to draw firm conclusions. Carbidopa reduces the amount of levodopa required by about 75 percent and, when administered with levodopa, increases both plasma levels and the plasma half-life of levodopa, and decreases plasma and urinary dopamine and homovanillic acid. In clinical pharmacologic studies, simultaneous administration of carbidopa and levodopa produced greater urinary excretion of levodopa in proportion to the excretion of dopamine than administration of the two drugs at separate times Pyridoxine hydrochloride (vitamin B6), in oral doses of 10 mg to 25 mg, may reverse the effects of levodopa by increasing the rate of aromatic amino acid decarboxylation. Carbidopa inhibits this action of pyridoxine. ... Studies show that peripheral dopa decarboxylase is saturated by carbidopa at approximately 70 to 100 mg a day. Patients receiving less than this amount of carbidopa are more likely to experience nausea and vomiting. The above information is not the whole story, of course. I do believe there is need to study the potential problem of over-dosing with carbidopa. I have found no report of such a study - and have brought this subject up several times on this list. Anecdotally, several have reported feeling better when the reduced their intake of carbidopa to the 70 to 100 mg. per day "saturation" dosage range. The "not appreciably metabolized" comment at the end of the Lodosyn (carbidopa) information means that build-up might occur. One can conjecture that extremely high concentration in the blood could produce some entering the brain - or some peripheral nerve anomaly. -- Ron Vetter 1936, 1984 PD dz ... "money is coined liberty" ... Dostoevsky e-mail: [log in to unmask] http://www.ridgecrest.ca.us/~rfvetter