More than a third of women who took part in a global drug survey reported being taken advantage of sexually while drunk or high.
The majority of the incidents took place in a private home by someone known to the person, the 2019 global drugs survey found.
Some 29.3% of women said they had been taken advantage of, with 8% saying it had taken place in the last 12 months.
Almost all respondents who said this happened within the previous year did not report the incident to police, with 43% failing to because they felt partly responsible.
Sex and drugs researcher Alexandra Aldridge said people can feel reluctant to use the words sexual assault because they may believe their experience is less valid if they are intoxicated.
The Royal Holloway PhD student, 26, said: “Our findings really show that we need to move away from victim blaming – telling women to change their behaviour clearly can only go so far in preventing sexual assaults.”
She added: “Clearly people are feeling responsible in some way for their actions, and I think a lot of people who experience harassment or being taken advantage of can really relate to that.
“‘To what extent does my experience count as sexual assault when I have these feelings of guilt and responsibility?’ – that definitely acts as a huge barrier to reporting.”
Researchers based in London questioned more than 120,000 people across the world between October 29 and December 30 last year.
Overall, 18% of men, women and people who identified as a different gender who took part said they had been taken advantage of while intoxicated.
The incidents involved unwanted kissing, oral sex, sexual touching and penetration.
Almost nine in ten of the encounters in the last year involved alcohol (87.8%), with 35.5% involving alcohol and drugs, cannabis being the most common.
Two thirds took place in a private house, while 69.5% involved someone they knew in some capacity.
More than a quarter (25.8%) of those who reported being taken advantage of said they had given consent to initiate sexual activity, but later changed their mind.
Ms Aldridge said the findings show the need to reframe the idea of consent as a process rather than a one-off.
She also warned it was important not to conflate consent with wantedness, with some people agreeing to activity “because they don’t feel able to say no, they feel under threat or they think something negative might happen to them if they say no”.
The report said: “The debunking of such rape myths has important implications for educational messages aimed at preventing intoxicated sexual violence.
“Such messages should not rely solely on strategies to avoid being raped.
“Instead, messages should promote ethical sexual behaviour in which individuals are encouraged to consider the effects of alcohol and/or other drug intoxication on their own and others’ feelings around sexual activity.”