Wellington-Gueph-Dufferin Public Health following eight overdoses in the last six days related to purple and clear/white fentanyl.
"There are also concerns about an extremely potent black fentanyl," said Public Health.
Overdose symptoms include atypical, erratic body movements.
"Carry naloxone. Never use alone. Start low and go slow. Use the Consumption and Treatment Site at Guelph Community Health Centre," says the alert issued late afternoon on Christmas eve.
The Consumption and Treatment Site is open through the holidays, from 9 a.m. to 4 p.m., including Christmas Day, Boxing Day and New Year's Day.
Public Health offers tips that include never using alone, carrying naloxone and "going low and slow."
