In horses, doxycycline is not particularly well absorbed following oral administration, but it does have good tissue penetration [39]. It is highly protein bound in the plasma and has a relatively slow rate of elimination. IV administration of doxycycline to horses results in cardiovascular collapse and death regardless of how slowly it is administered. Most tetracyclines are eliminated unchanged both in the bile by hepatic mechanisms and in the urine, although minocycline (and probably doxycycline) is primarily eliminated in the bile. Enterohepatic cycling, the reabsorption of drugs eliminated in the bile, can occur.