Fentanyl test strips don't always detect the presence of the deadly opioid and could give drug users a false sense of security, Health Canada is warning.

In an advisory issued Wednesday, the department said its drug analysis service did a "preliminary study" to compare "a fentanyl test strip product" used in communities against its own more sophisticated laboratory tests.

Demand for commercially produced test strips has boomed as recreational drug users and harm-reduction workers try to stop the rising number of deaths due to street drugs laced with fentanyl.

The Health Canada study compared fentanyl strip test results with its professional laboratory test results on 70 samples of drugs. Most of the samples "contained chemically similar drugs (fentanyl and analogues with similar chemical profiles, such as carfentanil)," department spokesman Eric Morrissette told CBC News in an email Wednesday evening.

In 63 of the samples, the test strips "accurately identified that opioids were present," he said.

But in three samples, the fentanyl test strips gave false negatives, indicating no opioids were present, when the lab tests indicated they were.

The strips showed false positives for opioids in two cases, and gave "invalid results" in two others.

Although Morrissette cautioned that the results are "very preliminary" and that the sample size was very small, he said the fact that "there is a possibility for false negatives in a small number of instances ... suggests more research on this issue is needed."

The risk of a false negative is particularly concerning, because it could "lead to a false sense of security which may result in overdose or death," Health Canada's advisory said.

"This is particularly true for people who may choose to use drugs alone or without visiting a supervised [drug] consumption site where emergency help is immediately available."

Health Canada is 'urging Canadians to treat all illegal drugs as potentially contaminated,' and to take precautions if using drugs. (CBC)

The department "is urging Canadians to treat all illegal drugs as potentially contaminated," and to take precautions if using drugs, including never using them alone, going to a supervised consumption site whenever possible, and ensuring naloxone (a drug used to reverse overdoses) is available.

In its next phase of assessing fentanyl test strips, Health Canada plans to use them with a greater variety and number of drug samples.

Morrissette was unable to immediately provide the brand name of the fentanyl test strip product that was studied, or specify what kind of drugs were tested.

But regardless of the brand, "Health Canada has not licensed any drug test strips as medical devices intended for drug-checking of illegal drug samples before consumption," Morrisette said.

However, provincial and territorial health ministries have the authority to distribute fentanyl test strips as a harm-reduction measure.

In British Columbia, clients at the supervised consumption site Insite have been allowed to use the strips to test their drugs. Drug users are not permitted to use them off-site in private homes or in the community. The Ontario government also announced earlier in the fall that it would provide fentanyl testing strips "at all current supervised injection services and pop-up sites and will be evaluated for further distribution."