SAN DIEGO (KGTV) -- The San Diego County Sheriff’s Department is warning San Diegans about a mysterious and possibly deadly counterfeit drug being traded on the streets.

The department says they believe the pills have caused four overdose deaths in San Diego County over the last 36 hours. The deaths happened in Valley Center, Santee, Poway and Lakeside.

“It’s being traded and sold on the streets with the promise of a painkiller high, but no one really knows what exactly is in the drug which makes it so dangerous,” the department said.

Authorities say the pills are blue with one side marked by an "M" while the other side is stamped with a "30." The department says the pills are often called "M-30s" or "blues."

While the department doesn't know exactly what's in the pills, investigators say these type of pills often contain fentanyl or carfentanil.

Anyone with information is urged to call the Crime Stoppers anonymous tip line at (888) 580-8477.

If you or someone you know is struggling with a drug or alcohol addiction, the department says you can call the County Crisis Hotline at (888) 724-7240.