The seizure of illegal drugs and counterfeit medications has triggered a warning by Queensland Health as police investigate two deaths potentially linked to the substances.

Health authorities say tablets found during various investigations in 2023 have tested positive to protonitazene, a strong opioid that can be fatal even in small amounts.

Some tablets such as Xanax, which is not commercially available in Australia, appear to be branded as genuine medications.

People have been warned not to consume counterfeit or altered medications and to tell authorities if they are aware of any.

"Protonitazene can lead to respiratory failure, loss of consciousness, coma and death, even if taken in small quantities," said Associate Professor John Allan, Queensland Health's executive director of mental health, alcohol and other drugs.

"Many of the counterfeit tablets appear genuine and are in labelled Xanax bottles. Unsuspecting users may not notice they are counterfeits.

"Nitazenes, including protonitazene, are a group of synthetic opioids that can be as strong or stronger than fentanyl. They have been detected in Queensland and other states and territories in various forms including tablets, powders and liquids.

"This highlights why people should not source substances illegally. You could unknowingly be taking a potentially fatal substance."

Symptoms of opioid overdose include shallow breathing, slow or erratic pulse, confusion, small pupils, unresponsiveness, and bluish or greyish skin from poor circulation.

Queensland Police confirmed they were preparing reports for the coroner in relation to the sudden deaths of two people this year that "may be related to an illegal substance".

"As the matter remains before the coroner, no further information is available," a representative said.