CYP2D6

CYP2D6 (cytochrome P450 2D6) is an enzyme produced mainly in the liver that breaks down a wide range of drugs and other foreign compounds. It is one member of a large enzyme family — the cytochrome P450s — collectively responsible for clearing the majority of pharmaceutical substances from the body.

What makes CYP2D6 unusually significant is its genetic variability. The gene encoding it is highly polymorphic, meaning individuals carry different versions that produce enzymes with very different activity levels. This variation divides the population into broad metabolizer phenotypes: poor, intermediate, normal (extensive), and ultra-rapid metabolizers.

How it works · its role

The enzyme works by chemically modifying its target molecules — typically adding an oxygen atom — so that they become more water-soluble and easier for the body to excrete. In the process it can convert a drug into an active metabolite, inactivate it, or produce a toxic byproduct.

Poor metabolizers carry variants that produce little or no functional enzyme. Drugs that rely on CYP2D6 for clearance build up in their bodies, producing stronger and longer-lasting effects. Ultra-rapid metabolizers carry extra gene copies and clear the same drugs so quickly that standard doses may have little effect.

A further complication is phenocopying: even someone born a normal metabolizer can be temporarily converted into a poor one if they take a drug that strongly inhibits CYP2D6. Several commonly used substances do exactly this.

Relevance to substances & effects

CYP2D6 is the primary metabolic route for a notable range of psychoactive and psychiatric substances. Codeine, tramadol, and oxycodone are converted by CYP2D6 into their more active forms; in poor metabolizers these prodrugs may produce little analgesia, while in ultra-rapid metabolizers the rapid conversion can push active-opioid levels to dangerous heights.

Dextromethorphan (DXM), a dissociative found in many cough medicines and used recreationally, is metabolised by CYP2D6 into a distinct active metabolite. Poor metabolizers experience a pharmacologically different DXM effect than normal metabolizers, a difference that can be pronounced at recreational doses.

MDMA is both a CYP2D6 substrate and a potent inhibitor of the enzyme. After an initial dose, the enzyme becomes significantly suppressed, slowing metabolism of any subsequent doses taken in the same session. This is one reason why redosing MDMA does not simply double the effect — the pharmacokinetics shift mid-session.

Many antidepressants interact with CYP2D6 in both directions. Fluoxetine and paroxetine are strong inhibitors, meaning they can substantially alter how other CYP2D6-dependent drugs behave in someone taking them. Tricyclic antidepressants and several antipsychotics are themselves CYP2D6 substrates, making their plasma levels sensitive to both genetic phenotype and co-administered inhibitors.

Clinical · risk note

The clinical consequences of CYP2D6 variation range from therapeutic failure to toxicity. The clearest example is codeine: ultra-rapid metabolizers have experienced life-threatening opioid toxicity at doses considered safe for the general population, and this risk is now reflected in prescribing guidance in several countries.

Drug-drug interactions mediated through CYP2D6 inhibition are common enough that they are a routine consideration in psychiatric and pain medicine. When two substances that both depend on the enzyme are taken together, or when one inhibits it while the other relies on it for clearance, the effective dose of the second drug can rise sharply without any change in what is taken.