The effect of an increased ratio of carbidopa to levodopa on the pharmacokinetics of levodopa

Acta Neurol Scand. 1985 Oct;72(4):385-91. doi: 10.1111/j.1600-0404.1985.tb00888.x.


A randomized, cross-over study was designed to compare the effects of an increased ratio (from 1:10 to 1:4) of carbidopa to levodopa on the fate of levodopa and carbidopa in 11 healthy subjects. Four combinations of carbidopa/levodopa (10 mg/100 mg, 25 mg/100 mg, 25 mg/250 mg, 62.5/250 mg) were used. Plasma levodopa, carbidopa, dopamine and dopac concentrations as well as urinary excretions of levodopa and dopac were determined by a sensitive high-performance liquid chromatography with electrochemical detection after a single carbidopa/levodopa tablet. As the ratio of carbidopa to levodopa increased, there was a significant increase in apparent t1/2 and AUC values of levodopa. At the same time the urinary excretion of levodopa increased and that of dopac decreased. The plasma ratios of levodopa/dopamine and levodopa/dopac and the urinary ratio of levodopa/dopac also increased. There were less subjective side-effects in the 1:4 groups than in the 1:10 groups. It is concluded that increasing the amount of carbidopa in relation to levodopa may be beneficial and further clinical studies are clearly indicated.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial

MeSH terms

  • 3,4-Dihydroxyphenylacetic Acid / blood
  • 3,4-Dihydroxyphenylacetic Acid / urine
  • Adult
  • Carbidopa / blood
  • Carbidopa / metabolism*
  • Dopamine / blood
  • Drug Interactions
  • Female
  • Humans
  • Kinetics
  • Levodopa / blood
  • Levodopa / metabolism*
  • Levodopa / urine
  • Male
  • Random Allocation


  • 3,4-Dihydroxyphenylacetic Acid
  • Levodopa
  • Carbidopa
  • Dopamine