02.03.6 Pharmacologically Active Metabolites

Some drugs produce pharmacologically active metabolitesMorphine is a pharmacologically active metabolite of heroin.  Morphine acts as an agonist at a particular binding site, the opioid μ-receptors, to cause pain relief.  Stimulation of the opioid μ-receptors also underlies the euphoric effects of morphine, and gives its potential for abuse.  When morphine is given orally, it undergoes extensive first pass liver metabolism, which lowers the concentration.  Morphine only enters the brain slowly.  Thus, only low concentrations reach the brain to have an effect.

Heroin is diacetylmorphine, and is more lipid soluble than morphine.  Thus, heroin enters the brain more readily than morphine.  Once, in the brain, heroin is metabolised to morphine, which gives the pain relief and euphoria.  For the same dose, much higher levels of morphine in the brain can be obtained with heroin than morphine.  This is a key factor for opioid addicts, who prefer heroin to morphine as their drug of abuse.