OTTAWA -- Police say a dozen people have been arrested in Ottawa in a drug investigation related to the trafficking of counterfeit pills containing the deadly opioid fentanyl.

Ottawa police say the investigation began last September and involved Ontario Provincial Police as the pills were also being dispersed to rural areas of eastern Ontario.

They say six men and four women were arrested Thursday morning when officers executed search warrants at a single family residence, two townhomes, two apartments and a storage locker in Ottawa.

Investigators say another man and a woman were arrested Thursday afternoon in Ottawa, and all 12 suspects are expected to appear in court on Friday facing drug and weapons charges.

They say officers seized counterfeit pills containing fentanyl, fentanyl powder, cocaine, and methamphetamine. Police also seized assault rifles, handguns, stun guns, a shotgun, ammunition, more than $130,000 in cash.

The arrests came after police and public health officials in Ottawa warned residents about counterfeit prescription medication that they suspected was linked to recent life-threatening overdoses in the city.

They said counterfeit pills can be manufactured to look almost identical to prescription opioids like Percocet and warned that illicit fentanyl has been detected in certain fake pills.

Fentanyl -- a drug prescribed for chronic pain management -- is roughly 100 times more potent than morphine and about 40 times stronger than heroin.

It produces a drug high but also depresses the body's rate of respiration, which can cause breathing to stop -- a dose of just two milligrams of pure fentanyl can be lethal.

Police have said many people are ingesting it unknowingly as the drug, which cannot be seen, smelled or tasted, is difficult to detect when laced into other drugs.

Ottawa officials are warning residents against obtaining drugs from non-medical sources -- like friends, online vendors or drug dealers. They say drugs should only be bought from a local pharmacy or medical professional.

Other police forces in the country have issued similar warnings about counterfeit medication.