Pharmacokinetics For the general pharmacokinetics of the tetracycline's, see Tetracycline Hydrochloride. Doxycycline hydrochloride is readily and almost completely absorbed from the gastrointestinal tract and absorption is not significantly affected by the presence of food in the stomach or duodenum. Mean peak plasma concentrations of 2.6 micrograms per mL have been reported 2 hours after a 200-mg dose by mouth, falling to 1.45 micrograms per mL at 24 hours. After intravenous infusion of the same dose peak plasma concentrations are briefly somewhat higher, but become very similar to those after oral administration following equilibration into the tissues. Its biological half-life varies from about 12 to 24 hours.
Doxycycline is more lipid-soluble than tetracycline.
It is widely distributed in body tissues and fluids. However, the majority of a dose of doxycycline is excreted in the faeces following chelation in the intestines. Nevertheless there have been reports of some accumulation in renal failure. Removal of doxycycline by haemodialysis is insignificant. Distribution Concentrations of doxycycline in CSF of patients with neurological manifestations of Lyme disease receiving doxycycline 200 mg daily by mouth ranged from 0.4 to 2.5 micrograms per mL at 4 hours after a dose.