Pakistan said on Monday it was investigating two local distributors of Swiss pharmaceutical company Roche's Avastin cancer drug after 12 diabetic patients injected with the drug went blind.

The Drug Regulatory Authority of Pakistan (DRAP) said the health authorities in Punjab, the most populous province, had launched the investigation into local use of the drug Avastin, which is licensed for use in Pakistan.

"Incidents of loss of vision in diabetic patients have been reported following treatment with Altered/Dispensed/Diluted Avastin injection," the regulator said in a statement.

Javed Akram, the province's Minister for Specialised Health, said police were questioning two men they believe to be the drug's distributors in the state.

"A high level committee has been constituted to probe the issue. A case has been registered against the distributor and his aide," Akram said.

Following the incidents, the DRAP instructed the importer to recall the suspected batches of Avastin 100mg injection, which it said had been created illegally.

"The sale/distribution of registered Avastin injection has been put on halt till verification of its quality through sampling and laboratory testing to safeguard public health," it said in a statement on its website.

On its website, Roche said Avastin was approved in more than 130 countries, including the US, to treat several types of cancer.

"Roche strongly condemns this criminal act of counterfeiting and is doing everything in its power to cooperate with the authorities to protect patients from counterfeits," said Roche in a statement to Reuters.

"In Pakistan, the vision loss from Avastin has been identified by the authorities as a case of contamination by a third party supplier," it added.

The regulator said in its statement that in the cases concerned Avastin had been used off-label, meaning outside its approved use, to treat diabetes-related eye conditions.

Cancer drug Avastin, when used at much lower doses, is similar to eye drug Lucentis and is used in many countries as a low-cost option to treat certain blindness-causing conditions.

In its statement, Roche said: "Avastin is not approved for any use in the eye. Counterfeit medicines pose a health risk to patients because their content may be ineffective and contain harmful ingredients."

Alam Sher, Punjab's deputy drug controller who filed the police complaint against the distributors, told Reuters that some companies buy Avastin and repackage it in smaller doses to make it more affordable for patients.

A sharp drop in the value of the local currency against the US dollar has inflated the price of drugs in Pakistan, many of which are either imported or based on imported ingredients. Record high inflation has also diminished the purchasing power of many people.