#075 · Drug of the Day 3-methylmorphine prodrug CYP2D6 → morphine Schedule II / III (combo) 2026-09-08

Codeine

A methyl group between you and morphine

3-methylmorphine · (5α,6α)-7,8-didehydro-4,5-epoxy-3-methoxy-17-methylmorphinan-6-ol · C18H21NO3 · MW 299.36 g/mol · CAS 76-57-3 · ChEMBL CHEMBL485

Codeine. Morphine with a methyl ether on the phenol. That ether is the whole drug: it cripples µ affinity until CYP2D6 O-demethylates it to morphine (#009). Your genotype writes the prescription. Poor metabolizers take a placebo with constipation; ultrarapid metabolizers have died on labeled cough-syrup or post-tonsillectomy doses — which is why the FDA contraindicated codeine in children after that surgery.

Primary targetµOR via morphine
MechanismCYP2D6 prodrug
Parent MOR Ki~300 nM – µM
Payloadmorphine (~5–15%)
PM phenotypeno analgesia
UM phenotypeoverdose risk
AlsoUGT → C6G
01 · Mechanism of Action

The Methyl Ether Is a Pharmacogenetic Fuse

Codeine's 3-methoxy group wrecks the MOR phenol hydrogen bond that morphine uses. Parent codeine is a weak µ ligand; clinically relevant analgesia is morphine produced by CYP2D6 (typically ~5–15% of the dose). A parallel UGT2B7 arm makes codeine-6-glucuronide, which has some µ activity but does not rescue poor metabolizers.

CYP2D6 ultrarapid metabolizers (extra gene copies, commoner in some North African and Middle Eastern populations) convert enough codeine that labeled doses have been lethal in children and in breastfed neonates of UM mothers. Poor metabolizers (~5–10% of Europeans) get antitussive/placebo pharmacology and still get constipation. This is the same 2D6 story as tramadol #021, with morphine instead of M1.

① Parent is a weak agonist

3-O-methylation drops MOR affinity by orders of magnitude versus morphine. Do not think of codeine as 'weak morphine already bound.'

② CYP2D6 O-demethylation

The fuse. PM: almost no morphine. EM: modest. UM: potentially toxic morphine exposure from 'therapeutic' codeine.

③ Morphine payload

Once formed, the ligand is #009: full µ agonist, M3G/M6G, respiratory depression, naloxone-reversible.

④ Codeine-6-glucuronide

Major metabolite via UGT2B7; some µ activity. Not enough to make PMs responders.

⑤ Antitussive

Cough suppression occurs at doses below typical analgesia and is not purely µ — one reason OTC/combo history is so messy.

⑥ Pediatric boxed history

Post-tonsillectomy deaths in UM children; codeine contraindicated in that setting. Genotype is not optional trivia.

Codeine → CYP2D6 O-demethylation → morphine (analgesia · apnea)
UM genotype → extra 2D6 copies → labeled dose can be an overdose
02 · Pharmacokinetics

Genotype Is the Dose

Oral Tmax ~1 h; t½ ~3 h for parent, morphine then follows its own kinetics. CYP2D6 is the activation step; CYP3A4 N-demethylation to norcodeine is a minor inactivation arm.

Oral bioavailability~50% (variable)
Tmax~1 h
Parent T½~3 h
ActivationCYP2D6 → morphine
Fraction to morphine~5 – 15% (EM)
UM riskmuch higher morphine AUC
UGT2B7codeine-6-glucuronide
CYP3A4norcodeine (minor)
03 · Combinations and the Purple-Drank Problem

Promethazine Does Not Make This Safer

Codeine/promethazine ("lean") adds an antihistamine/anticholinergic sedative on top of a pharmacogenetic opioid. Acetaminophen combinations add hepatotoxicity when people dose by opioid. The UM story is the same fuse tramadol has: the liver decides whether today's tablet is a placebo or a full agonist.

04 · FlexAIDΔS · Shannon Entropy Analysis

A Pharmacophore With a Cap On

FlexAIDΔS · Entropy Commentary

PDB 5C1M is active-state MOR + BU72. No codeine co-crystal is needed to tell the story: the 3-methyl ether is an entropic and enthalpic mismatch for the MOR phenol pocket. Hydrolysis to morphine restores the hydrogen-bond donor, collapsing the pocket the way #009 already described. The interesting Shannon term is metabolic — a polymorphic enzyme metering how much of the real ligand appears.

05 · Harm Reduction

The Fuse Sits in CYP2D6

Poor metabolizer: no pain relief. Ultrarapid: labeled dose may stop breathing. Naloxone still reverses the morphine.

Ultrarapid metabolizers

  • Extra CYP2D6 copies → toxic morphine from 'normal' doses
  • FDA: contraindicated in children after tonsillectomy/adenoidectomy
  • UM nursing mothers have intoxicated infants

Poor metabolizers

  • No analgesia — do not 'just give more' without recognizing the genotype
  • Constipation and some cough suppression can still happen

Combinations

  • Acetaminophen ceiling / liver failure if redosing combos
  • Promethazine + codeine: stacked sedation; not a harmless cocktail
  • Benzos/alcohol: same fatal synergy as any opioid

Practice

  • Naloxone reverses the morphine payload
  • Prefer non-codeine analgesics in kids; consider 2D6 when 'codeine doesn't work'
FATAL COMBINATIONS: CYP2D6 ultrarapid metabolizer + labeled codeine (especially children post-tonsillectomy, or breastfed neonates of UM mothers) → morphine overdose. Codeine + benzos/alcohol/promethazine → respiratory arrest. Acetaminophen combos → delayed hepatic failure.
3D · Active-state MOR (morphine payload pocket) PDB: 5C1M
Loading structure from RCSB…
Receptor (refined cartoon)
Contact residues
Ligand (ball-and-stick)
Structure: 5C1M — active-state MOR + BU72. No codeine co-crystal exists; this is the pocket morphine (the payload) occupies. Codeine itself is a poor MOR ligand until CYP2D6 unmasks the phenol. Rotate · scroll to zoom · right-drag to translate.
View on RCSB →

Receptor Binding Affinities

prodrug → morphine
Target Affinity Rel. Action
MOR · codeine
parent 3-methylmorphine
Ki ~300 nM – µM
weak agonist
Weak
MOR · morphine
CYP2D6 payload
Ki ≈ 1.8 nM
see #009
Full agonist
C6G
codeine-6-glucuronide
some µ activity
UGT2B7 major metabolite
Partial/weak
CYP2D6
activation enzyme
polymorphic
PM null · UM extra copies
Pharmacogene
Parent codeine MOR affinity is weak (ChEMBL CHEMBL485). Clinical µ effect tracks CYP2D6-formed morphine. PDB 5C1M is the morphine/BU72 active-state surrogate.