Philippine president Rodrigo Duterte has bragged about killing people and admitted being a drug abuser in comments his critics will use to question his fitness for office.

The 72-year-old former provincial mayor, whose crackdown on illegal drugs has left almost 6000 people dead, admitted over-using an addictive pain-killing drug to relieve pain.

Mr Duterte told a business forum in the presidential palace that he started taking the opioid analgesic drug fentanyl, which is usually prescribed for cancer patients, despite never being diagnosed with cancer.

He admitted his doctor told him to take just a quarter of a square fentanyl pill but he used to take two of them before being warned he was abusing the drug.