This study examined the effects of administering ferrous sulphate 325 mg with Sinemet (100/25 tablet) on levodopa and carbidopa bioavailability and on signs of Parkinson's disease in nine patients. 2. Ferrous sulphate ingestion with Sinemet resulted in a decrease in levodopa area under the curve (AUC) of 30% (P less than 0.01) and a greater than 75% decrease in carbidopa AUC. Despite a strong relationship between reductions in levodopa AUC and reductions in Sinemet efficacy (r = 0.83, P less than 0.01), the average reduction in Sinemet's efficacy associated with ferrous sulphate did not achieve statistical significance (P = 0.055). 3. Chemical studies indicate that iron forms chemical complexes with carbidopa in a similar manner to levodopa and is a likely mechanism for the drug interactions. 4. AUC when a Sinemet tablet is taken concurrently with a ferrous sulphate tablet appears to be clinically significant in some but not all patients. The clinical significance of repeated ingestion of ferrous sulphate with Sinemet requires further studies.
