Phase 1 Metabolism
Phase 1 metabolism can involve reduction or hydrolysis of the drug, but the most common biochemical process that occurs is oxidation. (Oxidation is the chemical reaction that occurs when apples turn brown when exposed to the oxygen in air).
Oxidation is catalysed by cytochrome P450 enzymes and results in the loss of electrons from the drug (electrons are negatively charged subatomic particles).
The drug is now said to be oxidised and after phase 1 reactions, the resulting drug metabolite is still often chemically active.