Contaminated heroin is believed to be in use in Scotland

Users who have taken the drug reported passing out, becoming violently sick and swelling and infection around the injection area.

The Scottish Drug Enforcement Agency (SDEA) said the heroin was believed to have been circulating in the Fife, central belt and Tayside areas.

It appears more yellow in colour than normal and smells like diesel or acid.

Addicts who took the drug also experienced pins and needles in the arm, sweating and shaking.

Detective Superintendent Gillian Wood, of the SDEA, said: "The reports of potentially contaminated heroin is a worrying trend.

"These incidents illustrate the dangers associated with the misuse of illicit drugs, namely you can never be sure of their true contents.

These incidents illustrate the dangers associated with the misuse of illicit drugs Det Supt Gillian Wood

"In light of the symptoms described by some drug users, I would urge anyone feeling unwell as a result of using heroin to seek medical attention immediately."

A contaminated batch of heroin was behind an apparent outbreak of botulism among drug users in Scotland in October.

Four cases of the potentially-fatal infection, which is caused by a nerve toxin, were found in users in the Strathclyde and Grampian areas.

In 2000, 23 drug users in Scotland - 18 of them from Glasgow - were killed by a bug, which blackens the skin and causes liver and kidney failure because of contaminated heroin.