A constitutional court challenge has been launched over the federal government's decision to prevent doctors from prescribing heroin to addicts.

Providence Health Care, which operates St. Paul's Hospital in downtown Vancouver, says its lawsuit involves five patients who have been part of a clinical trial known as SALOME.

The SALOME trial examined a specific form of prescription heroin and operated under a federal exemption, but patients who left the trial are no longer covered under that exemption.

In a written statement to CBC News, the office of Health Minister Rona Ambrose said the federal government was forced to close loopholes that allowed "for the feeding of addiction under the guise of treatment." 

"Under the Special Access Program, Health Canada can approve emergency access to certain medicines for Canadians with rare diseases or terminal illness," the statement said, adding that this program "was not intended as a way to give illicit drugs to addicts."

Doctors in Vancouver had obtained federal approval to prescribe the heroin to 21 patients who left the trial, but last month Ambrose intervened when she introduced new regulations to close what she described as a "loophole" that allowed the drug to be dispensed.

Dianne Doyle, CEO of Providence Health Care, says the patients are extremely vulnerable and haven't benefited from other treatments such as methadone.

The lawsuit alleges the new federal regulations violate the patients' charter rights and it is asking the B.C. Supreme Court to throw out the revised federal rules.

With files from CBC News