Police in southern Ontario are sounding the alarm after cold medication was discovered in several boxes of Halloween candy handed out to children.

Durham Regional police launched their investigation upon reports from a young girl who found a tablet of medicine among her treats, according to a news release issued Friday.

The girl, Dhymone Williams, told CBC News she discovered the orange pill inside a small, sealed box of Smarties she received on Halloween night.

"I knew it wasn't a Smartie because I know that Smarties are round and they have chocolate in it, and that one I could see right through it," she said.

The pill was a Dayquil capsule, an adult cold medication manufactured by Vicks that can cause side effects in children, including insomnia, anxiety and irritability.

"I was shocked, surprised, disgusted, everything. I just didn't understand it all," said the girl's mother, Tammy Williams.

Four other children have discovered similar pills in their Smarties boxes, according to investigators. All the cases relate to a single neighbourhood in Pickering, Ont., around the Shay Drive and Beaton Way area.

Police are still trying to find the source of the medicine, and have questioned employees at the chocolate manufacturer where the Smarties were produced.

However, investigators say the investigation is focused on two streets in the Pickering neighbourhood.

"Someone may have tampered with the actual package after production. They may have bought it at a store, taken it home and done this to it afterwards," said Durham Regional Police spokesperson Dave Selby.

Residents of the neighbourhood said they were appalled at the idea.

"And this used to be a nice neighbourhood, but now when I hear this, it's scary," said Tony Conti.

Another woman agreed that the prospect of a local resident tainting children's Halloween candy was unsettling.

"I was actually surprised and a little disgusted that someone would actually do that and want to hurt someone," said Kayla Goncalves.

Authorities in Hamilton, Ont., issued a similar alert last Halloween after after a child received the smoking cessation medication Zyban in their candy bag.