Occasionally, the only postmortem samples available for analysis are contaminated with formaldehyde, either due to embalming prior to sampling or because analysis is carriedout only when formalin-fixed tissues retained for histological study are available. Formaldehyde reacts with several drugs of forensic interest that contain either a primary or asecondary amine group to form their N-methyl derivatives. We investigated the stabilityof3,4-methylenedioxymethampetamine (MDMA), 4-methylmethcathinone (mephedrone) and 3-trifluromethylphenylpiperazine (3-TFMPP) in formalin solutions using three different formaldehyde concentrations (5, 10 and 20%) and three different pHs (3.0, 7.0 and 9.5). Analysiswas performed using high-performance liquid chromatography with diode array detection to determine the percentage degradation of each drug over time, up to 60 days. MDMA, mephedrone and 3-TFMPP are unstable in formalin solutions, with the degradation rate increasing withincreasing pH. After 28 days in 20% formalin, pH 9.5, there remained 57% of the initial 3-TFMPP concentration,11% of the initial MDMA concentration and 4% of the initial mephedrone concentration. Forensic toxicologists should be aware that, when analyzing for these drugs in an embalmed body or in tissues stored in formalin solutions, the methylated formofthe secondary amine-containing drug could be a more useful analyte than the parent drug.