One enzyme family, an enormous job
Phase I reactions are dominated by oxidation, and the engine behind most of it is a family of enzymes called cytochrome P450 (CYP), embedded mainly in liver cells. They use oxygen and a helper electron supply to insert oxygen into the drug, typically creating an –OH group. That small change is enough to start the journey toward excretion.
There are dozens of CYP variants, named by family-letter-number, like CYP2D6 or CYP2C9. They overlap, but a handful do most of the work. The single most important for drugs is CYP3A4, which alone helps metabolize roughly half of all medicines on the market.
What oxidation actually produces
Most CYP products are more polar and less active — a step toward harmless waste. But two important exceptions shape clinical practice. First, the product can be an active metabolite that drives much of the drug's effect. Second, oxidation can yield a reactive metabolite: a chemically aggressive intermediate that can damage the very cells that made it, a cause of dose-related liver injury.
A classic example is paracetamol (acetaminophen). At normal doses it is safely conjugated, but a minor CYP pathway makes a reactive metabolite that the body normally neutralizes with glutathione. In overdose, glutathione runs out and the reactive metabolite attacks the liver — which is why the antidote restores glutathione.
Why the same dose acts differently
People do not carry identical CYP enzymes. Inherited differences — pharmacogenetic polymorphisms — sort patients into poor, normal, and ultra-rapid metabolizers. A poor metabolizer clears a drug slowly and may build up toxic levels on a standard dose; an ultra-rapid metabolizer may clear it so fast the drug barely works. This is a foundation of pharmacogenomics.
- Poor metabolizer: low enzyme activity → slow clearance → higher levels → more risk of toxicity at a standard dose.
- Normal metabolizer: the response the dosing guidelines assume.
- Ultra-rapid metabolizer: very high enzyme activity → fast clearance → low levels → possible therapeutic failure (or rapid activation of a prodrug).