Effect of exercise on pharmacokinetics.

The increasing popularity of sporting events, even for people on drug treatment, has raised the question of the interaction of exercise and pharmacokinetics. Exercise reduces splanchnic blood flow, but possible changes in the absorption of orally given drugs seem to be of minor clinical significance. Absorption from intramuscular, subcutaneou and transdermal application sites may be accelerated by exercise, possibly causing harmful consequences, e.g. in diabetics treated with insulin. Exercise or physical work increases the rate and depth of respiration thus increasing alveolar exchange of gases and vapours. Physical activity increases muscular blood flow and the binding of digoxin to muscular structures, with a simultaneous fall in the concentration of serum digoxin. Reduction in blood flow to adipose and other inactive tissues may delay the distribution of some drugs that are stored or removed by these tissues. The change from supine to upright position can affect the distribution of a drug. Exercise reduces the blood flow in the liver and deactivation of drugs with flow-limited (high clearance) hepatic metabolism such as nitrates and lidocaine. Metabolism of capacity-limited (low clearance) drugs, e.g. antipyrine, diazepam and amobarbital, is not influenced by exercise. Renal plasma flow, urine excretion rate and urine pH are also reduced by exercise. This is an important reason why the serum levels of drugs eliminated through the kidneys increase during physical stress. The changes in parenteral absorption and distribution volume of some drugs caused by exercise, as well as the short half-life of drugs, are properties resulting in altered therapeutic/toxic response in those drugs with a narrow therapeutic range.