An error at the Roy Romanow Provincial Laboratory resulted in incorrect test results being sent out for about 450 patients.
The tests were carried out between Oct. 30 and Nov. 28.
Health officials said an information technology error happened during screening samples for codeine and diphenhydramine (Benadryl).
Both can be considered drugs of abuse if used improperly.
Codeine is used to treat mild to moderately severe pain and diphenhydramine is an antihistamine used to relieve symptoms of allergies, hay fever and the common cold.
Officials said the error was promptly corrected and updated lab reports were sent to all doctors who had ordered the tests.
They added that they do not believe the error had any significant effect on patient care.
The Roy Romanow Provincial Laboratory is the only lab in the province that provides comprehensive drug abuse screening.
More than 75,000 tests are conducted annually at the lab.