Once the steady state level has been reached, the therapeutic blood levels persist for 12 hours. Theophylline is usually readily absorbed following oral administration. The extent of absorption is negligibly influenced by food. Theophylline is distributed in the extracellular fluids and uniformly to all tissues. The drug has a mean biological half-life of 5 hours in adults and 3.5 hours in children with great individual variability.
Theophylline is metabolized in the liver.
The enzymes involved in theophylline metabolism are unknown, but do not include xanthine oxidase. Serum uric acid concentrations are not increased during theophylline administration and the drug is not contraindicated in the presence of gout or allopurinol administration. Theophylline clearance is markedly increased in smokers, likely due to stimulation of the metabolizing enzymes. Not presently recommended for children under 12 years of age, as a dosage schedule in this age group has not been established. There is a marked variation in blood levels achieved in different patients given the same dose of theophylline.