Over 20 men fell ill or fainted after drinking the cough syrup in Shahdra Town of Lahore.
Four of them died instantly while eight more died in hospital.
At least seven men were being treated in hospital and several of them were in a serious condition, police said.
Police claimed most of those who fell ill after drinking the cough syrup were drug addicts.
Officials said they had registered a case against several dealers and two stores that sold the syrup.
Bottles of the same cough syrup were found in the possession of the men and the factory where the drug was made had been sealed, officials said.
Several stores were selling the cough syrup for Rs. 25 to Rs. 40 a bottle, they said.