why is l-dopa usually given in combination with carbidopa
Carbidopa/levodopa It is primarily used to improve the symptoms of Parkinson's disease but does not change the course of the disease.[1] It can take two to three weeks of treatment before benefits are seen.[2] Each dose then begins working in about ten minutes with a duration of effect of about five hours.[2][3]
Tetraethyllead In most industrialized countries, a phaseout of TEL from road vehicle fuels was completed by the early 2000s because of concerns over air and soil lead levels and the accumulative neurotoxicity of lead. The use of catalytic converters, mandated in the US for 1975 and newer model-year cars to meet tighter emissions regulations, started a gradual phase-out of leaded gasoline in the US.[5] The need for TEL was lessened by several advances in automotive engineering and petroleum chemistry. Safer methods for making higher octane blending stocks such as reformate and iso-octane reduced the need to rely on TEL, as did other antiknock additives of varying toxicity including metallic compounds such as MMT as well as oxygenates including MTBE, TAME, and ETBE.