Attempted fun with maths:

Last night I was watching Mock the Week, which I fully accept is unforgivable, and one of the news stories mentioned was a claim by police that one in twenty drinks in nightclubs is spiked. Eagerly I dusted down my calculator and got tapping. Assuming that all drink-spiking victims are women, and that everybody drinks two drinks each on average, that means one in five women drinks a spiked drink every night. Good work sturdy women of Britain, who seem to be largely able to consume spiked drinks and go home unmolested!
Also, in a nightclub which sees five hundred patrons a night there will be a thousand drinks, of which fifty will be spiked. Assuming that the prolific drink-spikers manage five Micky Finns a night each, that means that one night club patron in a fifty makes a concerted effort to drug someone else. Factor in the likelihood that most of these spikers are men and the figures become more mind boggling still.
Imagine my disappointment when a trawl of Google failed to turn up said report.

Even so, it is very hard to credit the idea that drug rape is remotely as common as it is perceived to be, though no doubt it is deeply horrible when it does occur. Only 2 percent of women who claim to have been molested with the aid of drugs showed any traces of narcotics in their systems other than alcohol or recreational drugs. Although I am not arguing for the matter to be ignored entirely, it does seem odd that a person of sinister intent would run the risk of introducing an illegal knock-out drop into somone's drink rather than just wait for someone else to suffer the inevitable effects of voluntery overindulgence.