Fentanyls

4-anilidopiperidines: a piperidine ring carrying an anilide nitrogen, a scaffold that tolerates substitution at almost every position, which is why the family keeps growing. All are μ agonists, most are lipophilic enough to act very fast, and potency across the family spans an enormous range, from the clinical members at one end to carfentanil, used for immobilising large animals, at the other. Some entries here are neither: 4-ANPP is a precursor and norfentanyl a metabolite, so a laboratory report naming them is describing chemistry, not what someone took.

This is the dominant adulterant risk in the opioid supply — fentanyl and its analogues turn up in material sold as heroin and in counterfeit pressed tablets, mixed unevenly. Onset leaves little time to respond, chest-wall rigidity can complicate rescue breathing, and some analogues outlast a dose of naloxone, so re-sedation after an apparent reversal is a known pattern rather than a surprise.