If you buy them on the internet, you can't necessarily trust the label. Nearly a fifth of all samples tested contained cutting agents and drugs like cocaine. As the report says, "Just because a substance is labelled 'legal' does not mean that it is legal. Neither is it a guarantee that it is safe...The contents of a package are probably not 'what it says on the tin'."
This summer, there has been growing anxiety because these legal highs are very easy to get hold of but seem, in some cases, to produce violent and unpredictable reactions.
In June, the Daily Mail carried the story of Katie Wilson, 19, from Deeping St James, Lincolnshire, who took the legal high Benzo Fury and then walked naked through a local branch of Tesco, kicking a police officer who tried to arrest her.
Fined £200 by magistrates, she said, "I'm so embarrassed about it, I'm just horrified that it made me become something I'm not...It is very dangerous and it can do you real damage – I don't want others to make the same mistake I did."
More seriously, Alex Herriot, 19, a student from Edinburgh university, died in June this year after taking Benzo Fury at the RockNess festival in Scotland.
In a statement, his family said, "Alex was not a habitual drug user. We know that young people dabble in drugs and Alex had been warned to steer clear. Please, please be aware that certain drugs can kill and please don't be the next youngster to leave their family bereft."
The best advice to teenagers, said David Liddell of the Scottish Drugs Forum in a report on Alex's death in The Guardian, is to avoid legal highs at all times.
"There's no easy solution to any of it," he said. "We know that the chemical laboratories of China and elsewhere probably have ten new compounds to bring on stream. It's a moving target. No one has yet come up a better way of dealing with this."
So what are we parents to do? Basically the answer, as always, is to find out the facts. The FRANK website is a good starting point. (Although I must admit that the long list of drugs I had never heard of made me wonder whether I'd been living on a different planet for the past 20 years.) Or you can ring them on 0800 77 66 00 for confidential advice 24 hours a day.
As FRANK says, the most important thing you can do is to talk to your teenagers. Remind them that just because drugs are legal, it doesn't mean that they're safe.
"They can contain a range of potentially dangerous chemicals, and their make-up changes all the time, so you can never be sure what's in them and what effects they'll have."
Alcohol can be dangerous. But at least we've had thousands of years to get used to its effects. These new drugs are completely unknown.
Who knows what damage they can do?