Durham police are warning parents to carefully look over their children's Halloween candy after a young student found a DayQuil tablet in her sealed Smarties box.

The Grade 5 Pickering student went into her lunch bag on Wednesday to open the small box of candy she received while canvassing her Brock Road and Finch Avenue neighbourhood for treats. Among the Smarties, she found a tablet of the cold and flu medicine.

She immediately took the candy to an adult at her school who promptly called the police.

"The small Halloween-sized Smarties box appears to have been sealed and looked normal," Durham police said in a news release. "It was received at a home when the youth was 'trick or treating' in the Shay Drive and Beaton Way neighbourhood on Halloween night."

Children could experience harmful side effects if they were to swallow the small tablet, police said.