The bioavailability of drugs used to treat chronic diseases such as Parkinson's disease may have important implications for their clinical utility. Drugs with low bioavailability may cause a wide variation in clinical response between patients and even in the same patient. In addition, numerous factors - including gender, age, and gastric motility - may affect a drug's bioavailability. This is especially important in patients with Parkinson's disease, who develop response fluctuations as the disease progresses. Strategies that may improve the bioavailability of levodopa, the most efficacious medication for Parkinson's disease, include coadministering levodopa with carbidopa, a decarboxylase inhibitor, or with a catechol-O-methyltransferase inhibitor or using an alternative route of administration. Other adjunctive therapies used to treat Parkinson's disease have a wide range of bioavailabilities, which may also affect clinical outcomes. The bioavailability of adjunctive medications may be improved by the use of alternative formulations as well, such as orally disintegrating tablets or transdermal delivery. Considering bioavailability of a medication when prescribing drugs to treat Parkinson's disease may improve patient response and minimize adverse effects.