Buprenorphine undergoes extensive first-pass metabolism and has low oral bioavailability of 15 % (Cone et al. 1984). Bioavailability following sublingual administration of buprenorphine is higher, 50-60 % (Nath et al. 1999). After high sublingual doses of buprenorphine (8-24 mg), peak plasma concentrations are reached in 1 hour (McAleer et al. 2003, Ciraulo et al. 2006) and after low sublingual doses (0.4 mg) they are reached in approximately 3 h (Billingham 1981). Approximately two-thirds of a buprenorphine dose is excreted unchanged, and the rest is metabolized in the liver and intestinal wall. N-dealkylation of buprenorphine mainly via CYP3A but also CYP2C8 yields norbuprenorphine, and glucuronidation yields buprenorphine-3-glucuronide (Cone et al. 984). Norbuprenorphine is excreted in the urine after subsequent conjugation. 80-90 % of buprenorphine is excreted by the biliary system and enterohepatic circulation (Brewster et al. 1984)