Health officials say contaminated heroin may have caused at least a dozen recent cases of anthrax including six deaths in Glasgow.

The rising toll has prompted Health Protection Scotland, the national agency for protecting the public from infectious and environmental hazards, to issue a warning to all heroin users to stop using the drug, regardless of whether they inject it or take it by other means.

Officials told the Times of London that the risk to others, including immediate family members of those infected, remained low.

Police and doctors believe contaminated heroin or heroin mixed with a contaminated cutting agent could be responsible for the cases, which began to come to light three weeks ago.

Anthrax is an acute bacterial infection most commonly found in hoofed animals such as cattle, sheep and goats. It usually infects humans when they inhale or ingest anthrax spores, but cannot be passed from person to person.

"Evidence now suggests that potentially contaminated heroin may be in circulation in other parts of Scotland, not just the Glasgow area," said Dr. Colin Ramsay, consultant epidemiologist at Health Protection Scotland, referring to the country's most populous city.

Signs of infection include soreness at the injecting site developing into redness and then spreading into a black "scar." If not treated the infection can spread into the blood and other organs.

Gordon Meldrum, director general of the Scottish Crime and Drug Enforcement Agency, and drugs spokesman for the Association of Chief Police Officers in Scotland, said a major national investigation had been launched into the latest deaths.

"The deaths associated with anthrax are disturbing and are being treated very seriously by all the relevant authorities," he said.

The Associated Press contributed to this story