Detecting Antihistamines in Hair After a Single Dose

Scientists tested whether two common sleep‑aid drugs, diphenhydramine and cyclizine, could be found in hair after only one dose. The drugs are often sold without a prescription and can make people very sleepy, which is why they might be used in crimes that involve drug‑facilitated attacks. There was not enough information on how long these drugs stay in hair or at what levels they appear. Twelve adults took a single dose of each drug: 25 mg of diphenhydramine and 44 mg of cyclizine. Researchers collected hair samples before the dose, then again at one, three, five, and twelve months later. They used a highly sensitive lab technique that can measure as little as one part per trillion of the drug in a milligram of hair. The main drugs and their two major breakdown products were found in every sample taken up to five months after the dose.After one year, only a quarter of the hair samples still had diphenhydramine, while nearly two thirds still contained cyclizine. Measured concentrations ranged from zero to 610 pg/mg for diphenhydramine and up to 590 pg/mg for cyclizine. The decline in drug levels over time was significant. For diphenhydramine, average reductions were 43 % at two months, 70 % at four months, and complete loss by eleven months. Cyclizine showed similar trends: 40 % drop at two months, 64 % at four months, and almost total disappearance by eleven months. These results give clear reference ranges for what a single oral dose looks like in hair tests. They help forensic experts judge whether someone was exposed to these drugs in the past year.