A heroin user is being treated in a hospital in Cumbria for suspected Anthrax poisoning.
Blood tests are being carried out on the patient who was admitted to Carlisle's Cumberland Infirmary on Christmas Day.
He was suffering from skin lesions, a typical symptom of anthrax poisoning.
Health officials fear that contaminated batches of the class A drug are being circulated, after two users in Scotland died from anthrax poisoning.
Test results are awaited over a third death.
Det Insp Geoff Huddleston, of Cumbria Police, said: "I would like to reiterate that drug abuse is extremely dangerous at all times, it is particularly important for users to be vigilant at the moment with the threat of infected batches of heroin."
Dr John Ashton, director of public health at NHS Cumbria, said: "Anyone who injects drugs and experiences any unusual symptoms such as swelling or pain at the injection site should seek medical help immediately.
"Until we receive this patient's test results early next week, we cannot discount possible anthrax poisoning.
"I would stress that while injecting drug users need to be vigilant, the risk to the rest of the population is negligible.
"Even if these test results do show traces of anthrax, person to person spread is extremely rare and there is no significant risk of airborne transmission."