Episode summary: Two people, same drug, opposite outcomes. The answer is in your liver enzymes and brain receptors. This episode unpacks how CYP2D6, CYP2C19, and CYP3A4 variants can turn a standard dose into an overdose or a sugar pill — and why brain receptor polymorphisms like 5-HTTLPR and COMT Val158Met matter just as much. Show Notes A listener named Daniel posed a question many of us have wondered: why does sertraline save one friend's life and ruin another's? Same diagnosis, same medication, completely opposite outcomes. The answer lies not in the drug's personality, but in your genome. The biggest differences come from the cytochrome P450 system — specifically CYP2D6, CYP2C19, and CYP3A4, the liver enzymes that metabolize most psychiatric drugs. Genetic variants sort people into four categories: poor metabolizers (two non-functional copies, little to no enzyme), intermediate, extensive (normal), and ultrarapid metabolizers (extra copies that clear drugs too fast). For atomoxetine, a CYP2D6 substrate, poor metabolizers can see plasma concentrations five to ten times higher than normal — effectively an overdose at standard doses, causing jittery anxiety. Ultrarapid metabolizers clear it so fast they feel nothing. The same pill, two completely different experiences. For SSRIs like escitalopram and sertraline, CYP2C19 is key. Ultrarapid metabolizers may never reach therapeutic blood levels at standard doses, while poor metabolizers face safety risks — the FDA even limits citalopram to 20mg for poor metabolizers due to heart rhythm concerns. But metabolism is only half the story. Brain receptor variants like 5-HTTLPR (which affects serotonin transporter expression) and COMT Val158Met (which controls dopamine breakdown in the prefrontal cortex) further shape drug response. Someone with the short 5-HTTLPR allele has fewer serotonin pumps to block, making SSRIs less effective and more side-effect prone. COMT Val carriers, with faster dopamine breakdown, may respond better to dopamine-boosting drugs like bupropion. Pharmacogenetic testing exists — companies like GeneSight offer panels covering 15-20 variants, and a VA study found genetic testing cut time to effective treatment from 12 weeks to 6. But the evidence is mixed, and most psychiatrists still don't order a cheek swab before prescribing. For anyone cycling through meds, feeling like nothing works, this is the biology you've been missing. Listen online: https://myweirdprompts.com/episode/antidepressant-genetics-metabolism