Three pharmacies and a medicine factory in Pakistan have been closed by authorities after 13 people died from drinking cough syrup suspected of being toxic.

AP reported a further seven people were being treated in a hospital in the city of Lahore, and were lucky to survive.

The victims were mostly drug addicts from a low-income Shahdra Town neighborhood who took the cough syrup in an attempt to get high, police station chief Atif Zulfiqar told the London Telegraph.

"Some of the victims were found dead in a graveyard where addicts used to take different kinds of drugs," he said.

After falling ill from the syrup, they were rushed to hospitals in the city of Lahore but many could not be saved.

The incident happened over the past three days and the victims were aged from 15 to 45.

Khan said three men who owned pharmacies had been arrested for selling the allegedly toxic substance and their stores shut down. A factory had also been closed.

Khan told Reuters samples of the cough syrup were being tested at a laboratory to determine its ingredients and whether it was toxic.

The government has ordered an inquiry into the incident.