Durham Regional Police say the deaths occurred between Nov. 22 and Dec. 7.
Police say the evidence at each of the scenes "suggests the use of illegally-obtained narcotics," but that can't be confirmed until toxicology tests are completed.
Anyone using illicit drugs who experiences a "serious, physical illness" should get to a hospital immediately, or call 911, police say.
Police say it is too soon to determine if these deaths are related to a series of heroin overdoses that were reported in Toronto last month.
On Nov. 29, Toronto police reported responding to "a number of heroin overdoses" in a 24-hour period. They said it appeared that "a deadly form of heroin" was being sold.