Random drug testing in schools should not be encouraged across Britain until there is better evidence that it helps stamp out use, a charity report says.
Cannabis users could switch to harder drugs, the report says
It could even have the "perverse consequence" that cannabis users switch to harder drugs to avoid detection, the Joseph Rowntree Foundation says.
Both Labour and Conservatives back more use of random testing of pupils.
But the foundation said there was "little high-quality evidence" of its supposed benefits.
In the report Neil McKeganey, professor of drug misuse research at Glasgow University, argues that encouraging schools to introduce random testing would be "ethically complex".
It could also have "unintended adverse consequences", he said.
The report found random schemes tested only a small selection of pupils and were most likely to identify pupils who occasionally used cannabis - the most common type of drug use.
They were less likely to help in getting early support for those with more serious problems - like heroin users - because other drugs left the system more quickly.
Tests could also "undermine trust between pupils and staff", making problems harder to detect.
Prof McKeganey added: "It is difficult to judge the true likelihood of drug-testing being widely used in UK schools.
"Unlike the United States, no central government funding has been allocated for programmes.
"However, if random drug-testing programmes were to be piloted, there would be an obvious need to ensure that their impact was rigorously and independently evaluated."
Abbey School in Faversham, Kent, has already introduced random drugs tests, using mouth swabs.
As of two weeks ago, no pupils had tested positive, which the head teacher Peter Walker said indicated success.
The tests had worked as a deterrent and reduced the level of peer pressure to take drugs, he added.
For his study, Prof McKeganey reviewed the guidance on drug-testing programmes within UK schools provided by the Department for Education and Skills.
He also assessed existing research and theories on the subject.