Drug company Pfizer has continued to back an anti-smoking medication despite concerns about side effects, it has been reported.

According to Reuters, non-for-profit Institute for Safe Medication Practices issued a warning about smoking cessation drug Chantix.

The watchdog claimed more than 1,000 people had reported suffering from health problems after using the drug to a US health regulator during the first quarter of the year.

In addition, it claimed there had been 50 deaths reported in relation to Chantix.

However, Pfizer spokeswomen Kristen Neese said in a statement: "We stand by the efficacy and safety profile of Chantix when used as directed."

According to the Boston Herald, US regulator the Food and Drug Administration received nearly 21,000 reports of serious reactions to drugs, including over 4,800 deaths.

On top of Chantix, blood-thinning drug heparin accounted for a large portion of the complaints earlier this year.