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Detecting Antihistamines in Hair After a Single Dose
Monday, May 4, 2026
Diphenhydramine and cyclizine—common over‑the‑counter sleep aids—were shown to be traceable in hair for up to five months after a single dose.
These findings provide forensic experts with concrete reference ranges for detecting drug‑facilitated attacks.
Study Design
| Parameter | Detail |
|---|---|
| Participants | 12 adults |
| Dosage | 25 mg diphenhydramine, 44 mg cyclizine |
| Sampling Points | Pre‑dose, 1 mo, 3 mo, 5 mo, 12 mo |
| Analytical Method | Ultra‑sensitive technique (≤1 ppt per mg of hair) |
| Targets | Parent drugs + two major metabolites |
Key Findings
- Detection Window:
- Diphenhydramine: Detectable in every sample up to 5 mo.
Cyclizine: Detectable in every sample up to 5 mo; higher persistence at 12 mo.
- Concentration Ranges (pg/mg):
- Diphenhydramine: 0 – 610
Cyclizine: up to 590
Decline Over Time:
| Drug | % Reduction at 2 mo | % Reduction at 4 mo | Status at 11 mo |
|---|---|---|---|
| Diphenhydramine | 43 % | 70 % | Complete loss |
| Cyclizine | 40 % | 64 % | Near total disappearance |
- Long‑Term Persistence (12 mo):
- Diphenhydramine: ~25 % of samples positive.
- Cyclizine: ~66 % of samples positive.
Implications for Forensics
- Provides quantitative benchmarks for a single oral dose in hair testing.
- Enhances the ability to time‑stamp drug exposure within the past year.
- Aids in investigations of drug‑facilitated crimes where these agents are commonly misused.
Conclusion
The study demonstrates that both diphenhydramine and cyclizine can be reliably detected in hair for up to five months after a single dose, with measurable declines over time. These data supply forensic scientists with essential reference ranges to assess past drug exposure.
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