Alleged sexual assault cases, identified from the forensic science Northern Ireland (FSNI) database, which had toxicology assays carried out on either blood or urine samples, were examined for the years 1999 up to and including 2005. In 1999 there were 30 toxicology requests while in 2005 there were 51, representing a 70% increase. The percentage of cases containing alcohol, drugs or both increased from 66% in 1999 to 78% in 2005.The estimated average blood alcohol concentration remained broadly similar throughout the spread of years. It was found to be 218 mg% (milligrams per 100 millilitres) in 1999 and 217 mg% in 2005. The actual number of cases studied within the 12 h cut-off time rose from 9 in 1999 to 22 in 2005. The relationship between negative toxicology results and time delay between the alleged assault and forensic sampling was examined. This showed that between 44% and 74% of cases were found to have a time delay of > 12 h. Some of these cases may therefore represent false negative results.The presence of drugs, either alone or in combination with other drugs, doubled between 1999 and 2005. Increased identification was found with antidepressants, recreational drugs, benzodiazepines and analgesics, some of which were also associated with alcohol consumption.The findings are sufficient to cause alarm for the health and safety of certain individuals and their increased vulnerability to sexual assault in some social settings. Additionally, the legal implications of what constitutes valid consent needs to be considered further in the light of these findings, if attrition rates are to improve.