Poor metabolizer

Poor metabolizer (abbreviated PM) is a pharmacogenetic phenotype — a classification assigned to individuals whose genotype for a particular drug-metabolising enzyme leaves them with little or no functional enzyme activity. The term belongs to a four-point spectrum: ultrarapid, extensive (the statistical norm), intermediate, and poor.

Critically, the designation applies per enzyme, not per person. Someone may be a poor metabolizer for CYP2D6 and a normal metabolizer for CYP2C19. Population prevalence varies by both enzyme and ancestry: roughly 5–10% of people of European descent are CYP2D6 poor metabolizers, while CYP2C19 poor metabolizer status is substantially more common in East Asian populations.

The most clinically significant enzymes in this framework are members of the cytochrome P450 (CYP) superfamily — particularly CYP2D6 and CYP2C19, which together govern the clearance of a large proportion of psychoactive drugs.

How it works · its role

Drug-metabolising enzymes chemically transform substances so the body can excrete them. In a poor metabolizer, the relevant enzyme is largely absent or non-functional because both inherited copies of the gene carry loss-of-function variants. Substrates of that enzyme accumulate in the bloodstream rather than clearing at a normal rate.

For most parent drugs — those taken in an already-active form — this means higher plasma concentrations, a prolonged duration of action, and heightened dose-dependent effects at doses calibrated for the majority. The inverse holds for prodrugs, which depend on enzymatic conversion to reach their active form: slower metabolism yields less active compound, weakening or eliminating the expected effect.

The magnitude of the shift also depends on how exclusively a substance relies on the affected enzyme. When multiple metabolic pathways exist, the PM phenotype has less impact.

Relevance to substances & effects

CYP2D6 poor metabolizer status is especially relevant to opioid pharmacology. Codeine and tramadol are prodrugs that require CYP2D6 to convert them to their active opioid forms; poor metabolizers gain little or no pain suppression from standard doses of either. MDMA is partly metabolised by CYP2D6, and poor metabolizers may experience more intense or prolonged effects at an identical dose — a difference invisible from the outside.

CYP2D6 also governs clearance of many antidepressants, including several SSRIs and most tricyclic antidepressants. A poor metabolizer taking one of these drugs at a standard dose may accumulate plasma concentrations that correspond to a substantially higher dose in a normal metabolizer, amplifying both therapeutic effects and adverse effects.

CYP2C19 poor metabolizer status shapes the pharmacokinetics of some benzodiazepines — diazepam in particular — and SSRIs such as citalopram and escitalopram, again tending toward accumulation. Because standard dosing is calibrated for the extensive-metabolizer majority, PM phenotype is an invisible source of variability: the same tablet can behave like a different drug depending on who takes it.