Date-rape drugs may not be as prevalent as first thought, research has found.
Use of Rohypnol is not "widespread", said report
An Association of Chief Police Officers study has found many victims of sexual assault may have just been very drunk.
Of 120 cases from November 2004 to October 2005, it said 12 were suspected drug-assisted assaults - but none was linked to the date-rape drug Rohypnol.
Campaign group Women Against Rape said the study was "unhelpful" because it was already difficult for women to prove they had been assaulted.
The government said: "Rape is never the victim's fault."
A Home Office spokeswoman said the research added to the government's "understanding of the role of drugs and alcohol in rape".
"The government is working to put rape victims' needs first and to make it easier to bring rapists to justice.
"Rape is an appalling crime, which is never the victim's fault," she said.
She added that nearly £6.7m had been invested over the past three years in services for victims of sexual violence.
The study - thought to be the first of its size into drug rape - involved the Metropolitan, Greater Manchester, Derbyshire, Northumbria and Lancashire police forces as well as the Walsall area of the West Midlands Police.
The findings also revealed 119 of the 120 alleged victims admitted they had been drinking alcohol and forensic tests discovered evidence of alcohol in 52% of cases.
"In most cases, the alleged victims had consumed alcohol voluntarily and, in some cases, to dangerous levels," an Association of Chief Police Officers (Acpo) spokesman said.
"The report does not seek to deny or neutralise the incidence of drug-facilitated sexual assault but merely view the topic in the context of alcohol and other related issues."
And Det Ch Supt Dave Gee, co-author of the report, told BBC Radio 4's Today programme that 48% of victims said they had taken a combination of recreational drugs and prescribed medication, in addition to alcohol.
He said this "cocktail" of substances was likely to "debilitate" people.
"The most common method of spiking drinks is alcohol," he added.
The organisation's analysis also discovered 22 alleged victims were two to three times above the legal drink-drive alcohol limit.
Of the 120 people examined, 57 had traces of controlled or prescribed drugs in their systems, including cannabis in 20% of cases, cocaine in 17% and amphetamines (including ecstasy) in 9%, said Acpo.
In a total of 41 cases, one alleged victim had taken alcohol and illegal drugs, eight had taken alcohol and prescribed drugs and seven had consumed all three.
In all, the study concluded none involved Rohypnol and just two involved another date-rape drug, GHB. Ten were suspected drug-assisted assaults involving other sedatives or drugs.
Ruth Hall, of Women Against Rape, told Today the research "hasn't told us anything that we didn't know already and the effect is really just to try to put the blame on women again.
"There is a lot of moralism about women drinking that doesn't apply to men. People don't think that when a man gets drunk he should expect to be raped, but when a women drinks, it is heavily used against her in court."
Ms Hall told Today that her mind was "open" on the use of Rohypnol, but added that "the question is not what the substance is, but what the response is".