Authorities are investigating cough syrup believed to have killed 33 people in eastern Pakistan in the past three days, a government official said Saturday, the second time in recent months that suspect medicine is thought to have caused multiple deaths.

Also Saturday, an explosion ripped through a passenger bus while it was at a terminal in the southern city of Karachi, killing four people and wounding 40 others, police and hospital officials said. It's unclear if the blast was caused by a bomb or a gas cylinder exploding.

The deaths from the cough syrup occurred in Gujranwala and villages surrounding the city, said Abdul Jabbar Shaheen, the top administrative official in Gujranwala. Another 54 people are being treated at hospitals in the city who are also believed to have consumed the syrup. Those involved are thought to be laborers or drug addicts who drank the syrup to get high, said Shaheen.

Chemical samples collected from the victims' stomachs contained dextromethorphan, a synthetic morphine derivative used in cough syrup that can have mind-altering effects if consumed in large quantities, said Shaheen. It is being investigated whether the people affected by the syrup in Gujranwala drank too much of it, or whether there was a problem with the medicine itself, he said.

Twenty-three people died in the nearby city of Lahore in November after drinking bad cough syrup sold under the brand name Tyno. They were also described at the time as people who consumed the drug to get high.

Shaheen said the cough syrup involved in the incidents in and around Gujranwala was not sold under a single brand. He said there were some people in the city involved in the business of making cough syrup specifically to sell to drug addicts, and officials were trying to arrest the culprits.

Officials temporarily closed one Lahore-based pharmaceutical company whose cough syrup was found in the possession of some of those affected in Gujranwala and were investigating whether it caused any of the deaths, said Shaheen. (AP)