Concerns are growing over the safety of so-called legal highs following the publication of a report which questioned their safety.

The New Zealand Medical Association stated that herbal party pills, which are being used increasingly by students, have a range of serious side effects and should be avoided.

Party pills, which contain the ingredient benzylpiperazine (BZP) have become cheaply and widely available in the UK, often being sold at mainstream club nights.

There are fears that because the drugs are legal, people do not view them as potentially dangerous so they do not treat them with sufficient caution.

The pills, seen as an alternative to ecstasy due to the euphoric feelings that they induce, are often referred to as herbal highs. But NZMA chairman Dr Ross Boswell insisted that: "There is nothing herbal about them. BZP was developed as a drug to treat cattle for worms." The drug is illegal in several countries, including Sweden and Australia, and classed as a Schedule 1 drug in the US alongside ecstasy and heroin.

Side effects of BZP can include paranoia, insomnia, anxiety, palpitations, vomiting and even seizures, the association found. A third year Politics student at Bristol University said, "My friends and I tried these pills thinking that because they were legal they wouldn't be that potent. In fact they were much stronger than other illegal drugs we had tried in the past and we had a nasty experience."

Nitrous oxide, usually referred to as laughing gas, has also experienced a recent boost in popularity, and is available cheaply at a variety of student nights, and even at university balls. Hannah Duncan, a second year English Literature student at Nottingham Trent University, said, "I've seen a lot of people get messed up on it, but it's available at lots of club nights here now."

Dr Grant Blair, a registered GP who works at Imperial College, warned people about using nitrous oxide recreationally. "It's stupid and dangerous," he said, "Laughing gas is an anaesthetic. It was designed to put people to sleep and should be treated with extreme caution." The effects of laughing gas are short-lived, but some users experience blackouts and there is a danger of concussion as a result. Doctors in Britain have warned that prolonged use carries a risk of depression due to the drug's reduction of vitamin B12 in the brain.

Legal high vendor Mike Garcia of Laughterpromotions.com said: "As with all stimulants, party pills should not be mixed with alcohol. That is stated on the packaging of all the products I sell."

In New Zealand, where an estimated 5m BZP-based pills have been sold legally since 1999, it is illegal to sell the drug in licensed establishments. In England, however, the pills are available for purchase at club nights, where they could be mixed with alcohol. BZP may follow magic mushrooms, which were outlawed when a legal loophole was closed in July 2005.

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