Dihydrocodeine

DHC activates mu-opioid receptors in the pre-Bötzinger complex, depressing respiratory drive. Pre-existing respiratory insufficiency eliminates the safety margin between therapeutic and lethal respiratory depression. Fatal DHC overdose cases consistently show respiratory depression as the primary cause of death.

Mu-opioid-mediated respiratory depression causes CO₂ retention, which produces cerebral vasodilation and can worsen raised intracranial pressure. This is a class-level contraindication for all opioid agonists.

DHC produces reinforcement through mu-opioid-mediated disinhibition of dopaminergic neurons in the mesolimbic pathway (VTA → nucleus accumbens). Pharmacovigilance data from EudraVigilance and FAERS confirm abuse, misuse, and dependence signals. Historical data from Germany document compulsive DHC use patterns among opioid-dependent individuals since the 1960s.

Hepatic cirrhosis reduces oxidative metabolism of DHC, increasing oral bioavailability due to reduced first-pass extraction and decreasing overall clearance. This results in higher and more prolonged plasma concentrations, elevating the risk of respiratory depression and other opioid toxicity.

Glucuronide metabolites of DHC — particularly DHM-6-O-glucuronide, which has mu-receptor affinity comparable to morphine-6-glucuronide — are renally excreted. In severe renal impairment, metabolite accumulation produces prolonged narcosis and respiratory depression. Case reports document clinically significant toxicity requiring dose avoidance.

CYP2D6 ultrarapid metabolizers produce excessive dihydromorphine (DHM), which has approximately 70-fold greater mu-receptor affinity than the parent compound. This increases the risk of respiratory depression and overdose at standard doses. While DHC is less CYP2D6-dependent than codeine, the affinity differential makes UM status clinically relevant.

Neonatal and infant glucuronidation pathways are developmentally immature, leading to disproportionate DHC accumulation. Shimizu 2018 reported respiratory depression in a 1-month-old receiving prescribed DHC 2 mg/day, with DHC concentrations of 400 nmol/L measured 21 hours post-dose, attributed to developmental immaturity rather than CYP2D6 genotype.

Concurrent use of MAO inhibitors with opioids carries risk of unpredictable serotonergic and opioidergic reactions. No DHC-specific case reports were retrieved, but this is an established class-level contraindication for all mu-opioid agonists.

Co-administration with CNS depressants — including alcohol, benzodiazepines, GHB/GBL, and ketamine — produces additive or synergistic respiratory and CNS depression. All four interactions listed as lethal in the substance brief. DHC-related mortality data show polydrug involvement in fatal cases.

DHC reduces gastrointestinal motility through activation of peripheral mu-opioid receptors in the myenteric plexus. Freye 2001 demonstrated dose-independent prolongation of oro-cecal transit time at 60 mg and 120 mg controlled-release DHC. Pre-existing ileus or bowel obstruction is exacerbated.