All of the people stricken with the bacterial disease were believed to be heroin users, said a spokesman at Scotland's Health Protection Agency, who spoke on condition of anonymity in line with government policy. The agency said other cases were under investigation, but would not specify how many.
Experts say contaminated heroin or another powder-like substance used to dilute the drug may have spread the disease. Scottish officials warned that further cases might be detected beyond Glasgow, where 4 deaths have occurred. Another 2 people died near Dundee and Forth Valley.
Anthrax is an animal disease and regularly infects people in Africa and Asia, as well as parts of southern Europe. It can be treated with antibiotics if caught early, and does not usually spread from person to person. Left untreated, anthrax can be fatal.
"Heroin users all across Scotland need to be aware of the risks of a potentially contaminated supply," said Colin Ramsay, a consultant epidemiologist at Health Protection Scotland in a statement.
Only a handful of anthrax cases have been reported in the United Kingdom in the past decade. The disease mostly affects people exposed to contaminated animal hides and other animal products.
"It is highly probable that the contamination of heroin by anthrax is accidental," said Gordon Meldrum, director general of the Scottish Crime and Drug Enforcement Agency, in a statement. "Production processes (of heroin) can be basic and often be conducted in areas where there is contamination from animal carcasses or feces."
Scottish police said investigations into the anthrax deaths were ongoing.
"We are preparing reports for prosecutors," said David Steele, a spokesman for the Association of Chief Police Officers of Scotland. "Those who supplied the drugs are reluctant to talk, but we will investigate them fully."
Agencies working to help heroin addicts, like Turning Point Scotland, said they were scanning more than 120 users a day for signs of anthrax infection.
"We are checking everyone over," said Beverley Bell, a spokeswoman for the organization. "It usually reveals itself through little black dots or spots around the area of injection."
She said the agency was also passing out leaflets about anthrax to heroin addicts and asking them to warn other users of the risks.
Another bacteria outbreak struck Scottish heroin users in 2000, when an infected batch of the drug killed 60 people. In 2000, one person in Norway caught anthrax after injecting contaminated drugs.
In Scotland, authorities said heroin users, rather than the general public, were most at risk.
"Heroin users do need to be on their guard," said Ramsay. While anthrax spores can be spread in the air, officials said there was no evidence of that type of spread in this current outbreak.
"I would advise heroin users to stop using heroin and seek advice," Ramsay said.
He said any heroin users who noticed signs of infection - like redness or swelling around a needle injection site or a fever - should seek medical help immediately.
The World Health Organization said it was monitoring the situation, but did not expect the outbreak to spill over into the general public.