By Andy Tighe
Home affairs correspondent, BBC News
Drugs such as GBL (left) are increasingly used to commit date rape
So-called date-rape drugs are on the rise, according to the United Nations drug control agency's annual report.
The International Narcotics Control Board says tough measures against the best-known drug, Rohypnol, have worked.
But sexual abusers are turning to alternative substances subject to less stringent international controls.
It wants these placed on governments' controlled substances lists and for manufacturers to develop safety features such as dyes and flavourings.
Professor Hamid Ghodse, of the International Narcotics Control Board, said: "These drugs are used so as to tremendously reduce people's resistance to unwanted sexual activity and then subsequently they might not even remember what happened."
In the UK, ketamine, an anaesthetic, has been a class-C drug since January 2006, while the solvent GBL, or gamma-butyrolactone, was one of a number of "legal highs" that became class-C drugs last year.
But both substances also have legitimate uses, making it harder to keep them out of the hands of criminals.
In March 2009, London taxi driver John Worboys was found guilty of drugging and sexually assaulting a series of female passengers in the back of his cab.
Worboys gave his victims drinks laced with sedatives. Many of the women could only recall falling asleep in his taxi before waking up at home.
Others were left with flashbacks and vague memories of Worboys sitting beside them.
Drug traffickers are also increasingly using illegal pharmacies based overseas, the report says.
Orders are placed via the internet or telephone call centres, with no prescription or other authorisation required.
India is identified as one of the main sources of these transactions.
The report calls on individual governments to take appropriate action to prevent the misuse of modern communication technology.
The Vienna-based agency also comments on the widespread abuse of prescription drugs such as morphine, codeine and methadone, calling it a "hidden problem".
In some countries, more people are abusing these drugs than the combined number of people taking heroin, cocaine and ecstasy, it says.
In the US this amounts to 6.2 million people.