Gamma hydroxybutyric acid (GHB) is a natural neurotransmitter found in the brain in low concentrations. GHB has been used in general anaesthesia and is currently used to treat narcolepsy and alcoholism. The abuse of GHB, especially in date rape sexual assaults, has increased in recent years. GHB has a rapid rate of metabolism causing it to disappear quickly and criminal cases are often difficult to prosecute. This study is aimed at extending the window of detection of GHB beyond 12 hours by measuring the GHB-dependent changes in gene expression and finding robust surrogate markers of GHB exposure. Human monocytic leukaemia TH-P1 cells were treated with 10µM and 900µM concentrations of GHB and gene expression after 24h exposure to GHB was evaluated using Agilent SurePrint G3 Human gene expression 8x60K arrays. Microarray data were analyzed by GeneSpring GX software and differentially expressed genes were identified. The results show that 900µM GHB induces alteration in 2380 genes using P<0.05 and a fold change of >2 as criteria of significance and this number is reduced to 587 genes using P<0.001 and >2 fold change. Gene ontology (GO) enrichment analysis of the gene lists found in the GO cellular component the largest numbers of the altered genes were in the intracellular and membrane parts of the cell. In terms of GO molecular functions, the majority of altered genes coded for proteins and nucleotide binding sites and some of the altered genes affect the catalytic activity of steroid dehydrogenase enzyme. Quantitative real-time PCR analysis was carried out on the genes affecting steroid dehydrogenase activity to validate the microarray findings. The results show that GHB induces changes in gene expression in blood THP-1 cells and this information may be useful in finding markers for GHB exposure in forensic toxicology.