One in five recreational drug users in the UK has been taken advantage of sexually while vulnerable after alcohol or drug use. The finding is one of the starkest results from the 2013 Global Drug Survey, conducted in partnership with the Guardian, Mixmag and GT magazine, which polled more than 7,000 users.

The survey also suggests that the internet is starting to rival the backstreets as a place to buy illicit drugs, with 22% of users reporting they had bought drugs online.

But the extent to which drugs are apparently being used to take advantage of users will be of particular concern.

In the survey, 20% of drug users said they had been taken advantage of sexually after alcohol or drug use; in an answer to a separate question 14% said they had been given alcohol or drugs by someone they believed had the intention of taking advantage of them sexually; and 2.4% had said they had had sex without giving their consent as a result of being drugged without their knowing.

The survey used "being taken advantage of sexually" as specific wording on its questions in this area to include behaviour ranging from sexual assault up to and including rape. The particular phrasing was used to avoid language which may cause distress in respondents, and also to avoid language relating to specific criminal offences, to try to gain accurate responses.

The results suggest that both voluntary and involuntary drug use can leave users at significant risk of exploitation, and should be a significant factor in public health messages about drugs, according to Dr Adam Winstock, director of the survey and consultant psychiatrist and senior lecturer at King's College London.

"Being taken advantage of whilst under the influence of substances was surprisingly common in this sample," he said. "Our findings show that you don't need to have a drug or alcohol problem to experience drug-related harms and use often increases the risk amongst those already most vulnerable.

"Intoxication can also impair a person's ability to give informed consent, remove themselves from a risky situation or to take safe sex precautions."

More than 22,000 people worldwide completed the survey, which is conducted online. Participants are recruited via media outlets, including the Guardian, in different countries, so the sample is self-selecting and self-identified, but nonetheless gives an insight into the attitudes of users.

Women were three times more likely to report sexual assault as a result of drug use than men, while younger respondents were at significantly more risk: 16- to 20-year-olds were twice as likely to have been taken advantage of sexually than 21- to 30-year-olds, and seven times more likely than those over 30.

Winstock said the key message for those using recreational drugs was to be aware of the risk and look after others.

"The results also highlight how drug use within intimate settings can increase your risk of both opportunistic and planned sexual assault," he said. "Friends really need to keep an eye on each other; they're in the best position to spot risk and steer mates way from situations where intoxication and isolation can make sexual assault more likely."

The researchers have launched drugsmeter.com and drinksmeter.com alongside the survey results to allow users to compare their drug use anonymously with 45,000 other people, and to receive further information about risks.

The survey showed buying drugs online is becoming more mainstream. The online marketplace Silk Road is notorious as essentially an eBay for drugs: law enforcement cannot trace the computers of sellers or buyers, and transactions are made with the anonymous and untraceable online currency Bitcoin.

But the survey suggests this is just the most visible aspect of a much wider phenomenon. Just under a third of respondents had heard of Silk Road, while 14% had set up an account on the site and browsed its wares. However, only 3% said they had themselves bought drugs from the site – although a similar number had taken drugs a friend had bought on Silk Road.

Wider drug dealing online happens with less rigorous secrecy than Silk Road. Instead, it relies on obscure coded listings hidden among job adverts, private sales and suspiciously cheap properties to let: "flat for rent – £60 – ask for Charlie".

The findings also highlight the risks of alcohol and tobacco, the two most commonly used legal recreational drugs. Just under 2% of people who had drunk alcohol in the last 12 months reported having sought emergency medical treatment as a result of their consumption, compared with 0.4% of cocaine users, 0.3% of ecstasy users, and 0.2% of cannabis smokers.

Alcohol and tobacco were the drugs most people wanted to give up: 40% of smokers said they wanted to cut back, and 34% of drinkers said the same; among heroin users (albeit a small sample of just 65 users) only 26% said they wanted to consume less.

Inside the report

• Prescription addiction: this year's Global Drug Survey contained yet more evidence of a prescription drug misuse problem in the UK. More than half of those taking benzodiazepines (sleeping pills) said they had obtained them from a friend, not a doctor, while 35% said they had bought some from a dealer. Just under a third of benzoate users said they took the pills "to get high" – as did 20% of people taking codeine tablets, and 23% of people taking prescription opioids. 15% of people who had been prescribed these drugs in the last year said they had shared them, while 20% said they had taken at least one overdose.

• Drug driving: the UK is set next year to introduce roadside tests for drug use, alongside definitions of drug levels which "impair" driving and can lead to criminal prosecutions – but many drug survey respondents think police would be unable to tell if they were driving on drugs. Only 17% of respondents said police would "definitely" or "probably" be able to tell they were intoxicated if they were pulled over within two hours of smoking a joint of cannabis. The figure was similar for cocaine users, at 15%, but MDMA users were much likelier to think police would be able to tell: 41% said police would notice, against just 5% who thought they would not.

• Synthetic risk: so-called synthetic cannabis – herbs sprayed with chemical compounds which supposedly cause similar effects to cannabis – has not achieved anything like the popularity of its natural counterpart, but does have a small but growing user base, with just under 9% of respondents having tried it. However, those taking the drug may be running much higher risks than users of natural cannabis: while only 0.2% (1 in 500) of cannabis users reported seeking emergency medical treatment as a result of their drug use in the last 12 months, 2.2% (1 in 45) of synthetic cannabis users had done so.

• Not talking to Frank: despite a huge advertising push over several years, drug users still do not seem all that inclined to turn to government websites (such as Talk to Frank) for advice on drug use. Asked where they would send a friend for information on drug use, only 2% of respondents suggested such websites. Telephone advice lines did somewhat better, at 9%, while local drug services managed 20%. Perhaps surprisingly, a third of survey respondents said they would recommend their GP for advice on drug use, but it was overwhelmingly friends and family, at just over 60%, that won out as the premier information source.

• Mystery white powder: around one in seven (14%) of respondents said they had, in the last twelve months, taken a mystery white powder without knowing what it was – a similar level to last year, when the phenomenon was first examined. Among recent users of cocaine, speed or MDMA, this figure doubled to 28%. Alcohol was a huge factor contributing to people's willingness to try a mystery drug: 78% said they were intoxicated when accepting the powder. But perhaps most disturbingly for those worried about the consequences, 39% accepted the powder from a stranger or someone they did not know well.

• Supermarket-style pricing: drug dealers seem to be adopting a tactic born in the supermarket ready-meals market – tiered pricing. More than half of people who had bought MDMA pills said they had been offered "better quality" pills for a higher price – up to £20 a pill – and 64% elected to pay for the premium-quality offering. Cocaine users reported similar results: the typical price paid for cocaine was £50 a gram, but 65% of those who purchased were offered an upgrade, with an average price of £80 a gram. In this case, 60% of users elected for the pricier option.

• This article was amended on 19 April 2013. The original referred to benzoates where it should have said benzodiazepines.