Police and health authorities are still investigating a significant botulism outbreak which is suspected too have infected 21 drug users across Scotland since just before Christmas.

The presence of botulism, which can cause paralysis and even death if left untreated, was confirmed by lab tests in 19 of the 21 possible cases, all of whom were admitted to hospital.

Health Protection Scotland, which is working with NHS Boards and Police Scotland to try to contain the outbreak, said the source was believed to be heroin contaminated with clostridium botulinum spores. All of those affected had bought drugs in or sourced them via Glasgow, and injected them, but it is believed no contaminated heroin has yet been seized by police.

The bacteria, not directly harmful themselves, can produce highly poisonous toxins when deprived of air.

Symptoms of botulism poisoning can include blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth and muscle weakness. If left untreated the illness may progress to cause paralysis and death.

Treatment involves antibiotics and an anti-toxin held at five locations, in Glasgow Royal Infirmary, Raigmore Hospital, Inverness; Ninewells in Dundee; Aberdeen Royal Infirmary and the Western Isles Hospital at Stornoway. Stocks are now also being held at St John's Hospital Livingston in response to the increased demand.

Across the UK there were only two cases of botulism poisoning among heroin users in the whole of 2013, according to government figures.

While six further cases of botulism have been reported in Norway, HPS says there is no information at this point to suggest a link with the Scottish cases.

Dave Liddell, Director at Scottish Drugs Forum urged medics and those working with people who inject drugs to look out for those who might be affected. Unless people stop taking heroin, it is impossible to prevent the risk of infection, so catching cases early important, he said.

"This outbreak continues to cause concern. Obviously not injecting heroin and perhaps smoking as an alternative is a means of avoiding the risk of infection. For those who continue to inject, people who inject under the skin or into muscle, are at greater risk."

He added: "The geographical spread of the cases suggests that there is no single batch that we could hope would be seized by police. Our response, therefore, depends wholly on communicating with drug users and service staff.

"We have been working with colleagues in getting out messages to users and staff about what symptoms they should be looking for."

Anyone showing the relevant symptoms should attend A&E and tell staff they are a drug user, he said. " Although A&E staff have been informed, the symptoms are non-specific to botulism so a full history is important."