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Recently, some of the hair samples that we routinely analyse for drugs of abuse did not produce valid results for cocaine and metabolites. A series of very intense interfering peaks with ion fragments common to cocaine (CO), and benzoylecgonine (BE) were found to cover up the "cocaine" region of the chromatogram. In one of these cases the subject declared he had used a lotion containing Minoxidil in order to prevent hair loss. Starting from this observation we found that the interfering peaks belonged to four different TMS derivatives of Minoxidil. Minoxidil interference was further investigated by applying Tricoxidil(®), a Minoxidil solution, to the hair of CO-free volunteers and to a CO-positive hair strand dipped into Tricoxidil. Hair were analysed before and after treatment. In both cases interfering peaks were absent in the chromatograms of untreated hair and appeared in treated hair. In the CO-positive hair detection of CO, BE and internal standard was completely hindered after treatment with Minoxidil. Attempts to separate interfering peaks from CO and metabolites by modifying the temperature programme failed. None of the hair washing methods tested (methanol; dichloromethane; sodium dodecyl sulphate water solution, 1% w/v followed by methanol; phosphate buffer 0.1 M, pH 6 followed by methanol) succeeded in removing Minoxidil interference. However, a simple solution to partially overcome the problem was to dry up the derivatised extract, reconstitute it in methanol (in order to switch back Minoxidil derivatives to the native molecule), and re-inject it: owing to the higher polarity, underivatised Minoxidil does not interfere any more with the chromatography of CO, at the expense of the disappearance of BE and ecgonine methyl ester both producing TMS derivatives. This strategy was applied to four real cases where Minoxidil interference was recognised: in two of these cases CO was detected. The problem of Minoxidil interference on CO detection may be limited to procedures involving trimethylsilylation, which is probably the most commonly adopted derivatisation in laboratories performing hair analysis for drugs of abuse.