Health authorities are warning of the dangers of Ketamine, a powerful tranquilliser used on horses, which is becoming increasingly popular among young people as a recreational drug.

Some users have needed to have their bladders removed and must now wear catheters. Other users have suffered serious kidney problems, breathing difficulties, addiction, bouts of unconsciousness and trouble with urinating.

The drug also produces a heightened risk of heart attack.

Some users also end up with cocaine-style damage to the inside of their noses because the drug is often snorted as a powder. It can also be injected, taken as a pill or swallowed as a liquid.

Experts say ketamine is increasing in popularity partly because it is cheaper than cocaine and, as the purity of cocaine falls, gives a more reliable high. It sells for about half the price of cocaine, at £20 ($45) a gram in the UK, but can be obtained for as little as £5 a gram.

"The quality of heroin and cocaine is so poor that people are turning to ketamine, which is cheap and available," said Dr Chris Ford, the clinical lead for substance misuse management in general practice in the London borough of Brent.

Dr Angela Cottrell, a urologist at the Bristol Urological Institute, has studied the health problems caused by ketamine.

She saw her first patient with severe bladder problems in mid-2007 and has seen a growing number of cases since.

"About one-third of ketamine users develop severe problems with the drug. There's something about the way it's metabolised that is causing these problems," said Dr Cottrell.

"One of the most alarming things is that the long-term effects on the body are not known. We don't know if things get better over time or whether people will develop kidney failure in the long-term."

The damage to vital organs may be irreversible, Dr Cottrell warned.

Ketamine is a stimulant and a hallucinogenic. In 2007, Professor David Nutt, recently sacked as the chairman of the British Government's drugs advisory panel, published research in the Lancet which ranked ketamine as the sixth most harmful substance out of 20 studied.

It came behind heroin, cocaine, barbiturates, street methadone and alcohol, but ahead of cannabis and ecstasy, in 11th and 18th places.

The drug is known as K, Special K and, because of the youth of many users, "kiddie smack". The Addaction specialist drugs service in Lincoln sees about 200 children under 18 every year. In 2007, none said they used ketamine.

Between June and November 2008, one teenager said it was their main drug and six said it was their secondary choice, usually behind alcohol or cannabis.