The rate and degree of blood and absorption into the blood is dependent on the route of administration. Effective absorption from the oral route depends on both the chemical properties of the drug and the functional efficacy of the GI tract. Parenteral absorption is dependent on the extent of blood supply. The bioavailability of a drug is the most critical part of pharmacokinetics.
There are two main ways that heroin is metabolised; one of the ways is through the Hepatic First Pass. This is done via the removal of an acetyl group when taken orally. The other way is through injections. Heroin that is administered via this route will evade the Hepatic First Pass and will quickly cross the blood-brain barrier. This is because of the presence of an acetyl group that makes it more soluble to fat. Once, in the brain, the acetyl group is removed, and heroin is reduced to 3-monoacetylmorphine and 6-monoacetylmorphine.