Abstract Trihexyphenidyl (THP) is an anticholinergic agent with forensic toxicological interest. The stability of THP was studied in postmortem blood and urine samples at a concentration of 0.25 μg/ml under different storage temperatures. After solid phase extraction (SFE), THP was measured by gas chromatography. On day zero and at intervals over a 6 months period, there was no significant loss of THP at the storage temperatures –20° C and 4° C in the spiked and authentic samples. Blood and urine sam- ples stored at 25° C showed a maximum recovery loss (about 14%) of THP after 3 months of storage. This loss was considered a significant change and corresponded to a P value < 0.046. The study demonstrates that the analy- sis of blood and urine samples containing THP would pro- duce consistent results when they are stored for 6 months at –20 or 4° C and for 3 months at 25° C. Key words Trihexyphenidyl · Stability · Blood · Urine Introduction The knowledge of the fate of a drug in biological samples stored under different conditions is of prime importance for the correct interpretation of the result. There are many studies in the literature on the issue of sample storage and putrefaction on substances of forensic interest [1–10]. Also such literature is continuously enriched with the emergence of new drugs. Trihexyphenidyl (THP, benzhexol) is an anticholiner- gic drug used in the treatment of parkinsonism, particu- larly the tremor that is characteristic of the disease [11]. The chemical structure of THP is shown in Fig. 1. The pharmacokinetics of THP in humans have been evaluated [12–14] and it is rapidly absorbed after oral administra- tion, producing peak plasma levels at a mean of 1.3 ± 0.2 h [13]. The half life (t 1 / 2) of THP in plasma in normal vol- unteers was reported to be 10.1 ± 4.5 h after a 5 mg oral dose [12]. Hydroxy-THP was reported as the major metabolite present in plasma and urine and accounted for two-thirds of the THP present in urine [15]. The drug is of forensic toxicology interest due its frequent abuse [16–21] and a reported overdose [22]. In Jordan, the abuse of ben- zhexol has also been noticed with increasing frequency in recent years amongst Jordanian youths. In general, the abuse potential of THP is attributed to its euphoric and hallocinogenic properties [11, 23]. Symptoms of an over dose are manifested as drug-induced toxic psychosis and include euphoria, disorientation, hallocination and para- noid ideation, mydriasis, warm skin, dry mouth, tachycar- dia, ataxia, constipation and absent bowel sound [23]. There are no available data regarding the stability of THP in biological fluids. This article presents a study on the ef- fect of different storage temperatures on the stability of THP in postmortem blood and urine. Solid phase extrac- tion procedures were applied during sample preparations [24–30]. Material and methods Blood and urine samples An aliquot of a methanolic standard of THP was added to a glass container and evaporated to dryness at 40° C under a gentle stream of nitrogen. No breakdown products occurred during this process. The residue was reconstituted with blood obtained from cadavers within 24 h after death. The blood was tested for THP before re- constitution and after an initial quantification the THP concentra- A. H. Battah · K. A. Hadidi Stability of trihexyphenidyl in stored blood and urine specimens Int J Legal Med (1998) 111 : 111–114 © Springer-Verlag 1998 Received: 8 April 1997 / Received in revised form: 31 October 1997 ORIGINAL ARTICLE A. H Battah () · K. A. Hadidi Forensic Medicine and Toxicology Division, Faculty of Medicine, University of Jordan, Amman, Jordan Fig. 1 The chemical structure of trihexyphenidyl hydrochlo- ride