Image caption An inquiry has been launched into the deaths

At least 16 people have died in the Pakistani city of Lahore after drinking what police say was toxic cough syrup.

The factory producing the medicine has been sealed, and three people have been arrested, including a distributor of the medicine.

The deaths occurred between Friday and Sunday. Police say they were mostly drug addicts in Shahdra town area.

Cough syrup, which is sold as an over-the-counter medicine, is sometimes used by addicts to get high, police say.

"Some of the victims were found dead in a graveyard where addicts used to take different kind of drugs," local police chief Atif Zulfiqar told AFP news agency.

The chief minister of Punjab province has ordered an inquiry to submit a report to him within 72 hours.

The provincial health authority raided several pharmacies and seized stocks of the syrup - media reports say the medicine was not past its expiry date.

Samples of the medicine have been sent for testing. Doctors told the BBC those who died may have added something to the syrup, to increase their high.

In January more than 100 people died after taking contaminated heart medicine in Lahore.

The deaths were directly linked to a batch of faulty drugs given to cardiac patients at a government-run hospital.

The BBC's Orla Guerin in Islamabad says the sale of sub-standard and expired medicine is a common problem in Pakistan.