It is feared contaminated batches of the class-A drug may be available to users.
The unidentified male was admitted to Carlisle's Cumberland Infirmary on Christmas Day suffering skin lesions, a typical symptom of anthrax poisoning.
Two drug users in Scotland have already died from the disease, as has a third person who health officials believe may have been poisoned after using contaminated heroin.
Blood tests are being carried on the patient in Cumbria to determine the cause of his condition. The test results are due back early next week.
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 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."
Detective Inspector Geoff Huddleston of Cumbria Police said: "I want to reassure our public that we currently have detectives who are making inquiries in the Carlisle area and are working with the Cumberland Infirmary in connection with this incident.
"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.
"If you are a drug user and display any unusual signs or develop an infection, you should go straight to hospital immediately."
The two deaths in Scotland were at Victoria Infirmary and the Royal Infirmary, both in Glasgow.
The possible anthrax-linked death was also at the Royal Infirmary.
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