At least 100 people belonging to the Pakistan's Punjab province died after taking spurious heart drugs and more than 200 are in a critical condition in hospital, according to official reports.
"Nearly 100 patients have died due to a reaction to heart drugs," said Shahbaz Sharif, the head of the government in central Punjab province.
The victims were mostly poor patients who received free drugs from the state run Punjab Institute of Cardiology (PIC), he said.
The government said another 287 people had been admitted to hospitals in Lahore, Pakistan's second largest city, after taking the drugs.
Police have arrested owners of three pharmaceutical companies suspected of supplying the medicine.
"Police registered a case on Tuesday after reports of deaths of people due to reaction of drugs for cardiovascular diseases," police spokesman Haroon Rashid told AFP.
Initial investigations show that drugs supplied by local manufacturers were "sub-standard," police said.
The patients died due to a sudden drop in white blood cells, platelets and bone marrow damage.
The problem was first detected in December when contaminated drugs in at least one batch of medicine caused 23 deaths.
Sharif said samples of the suspect drugs have been sent to laboratories for tests in Pakistan, London and Paris.
"Action will be taken against those found guilty," he said.
Government official Khwaja Salman Rafiq said most of the deaths were due to "at least one of five medicines which PIC had been prescribing".
The medicine has since been removed from hospitals and stores, he told AFP.
Doctor Javed Akram, involved in the investigation, said 46,000 patients receive drugs from PIC every month. "On the basis of this we suspect the number of patients affected by sub-standard drugs may rise," he said.
Lahore the capital of Punjab, last year faced an outbreak of the deadly tropical disease dengue fever which killed more than 130 people.