Philippines President Rodrigo Duterte could face impeachment after admitting to killing alleged drug addicts when he was mayor of Davao city.

The former mayor told a business forum he used to hunt suspects on his motorcycle, shooting them on sight.

“In Davao I used to do it personally. Just to show to the [policemen] that if I can do it, why can’t you?” Mr Duterte said.

“I go around in Davao [on] a big bike and I would just patrol the streets and looking for trouble. I was really looking for an encounter to kill.”

Mr Duterte also admitted to abusing the drug fentanyl – an incredibly potent opioid – to deal with migraines and back pain.

He said his doctor prescribed him a quarter tablet each day, but the president began taking two tablets before being warned it was dangerous.

“When [the doctor] knew it, he made me stop … the first thing that you would lose is your cognitive ability.”

Philippine Senator Richard Gordon said the president’s remarks could trigger an impeachment.

“When he says that, he’s opening himself up, isn’t he? He said it, so the legal way is to go ahead and impeach him,” he said.

“I’m sure he knows how to defend that,” added Senator Gordon, an independent politician, who normally supports the president’s policies.

Since Mr Duterte was elected president in May, the war on drugs by his administration has killed closed to 6000 people, out of which over 2000 have died in police operations and the rest in extra-judicial killings.

The violent campaign against drug-trafficking has been strongly criticised by international and regional human rights groups, including the United Nations and the European Union.