President Rodrigo Duterte, who has a habit of chewing gum in public, even while meeting world leaders, has revealed he did it to ease pain from a spinal condition.

The fiery 72-year-old has upset traditional decorum by lacing his speeches with curses, chewing gum in public, and wearing casual clothes, sometimes dispensing with socks.

Duterte affirmed his gum habit during a speech to a private audience on Thursday, when he suggested how people should repay him for his public service.

"Just pay me chewing gum because it eases up the pain when I'm chewing," he said, according to a transcript of the speech released by his office Friday.

He suggested it had something to do with a "spinal" condition but did not elaborate.

"It eases a bit," he added.

Protocol-conscious Japanese officials were on faux pas alert ahead of a meeting with revered Emperor Akihito last year after a video of Duterte with Chinese President Xi Jinping showed him apparently chewing gum -- considered rude on such an occasion.

A photo of Duterte meeting with Cambodian King Norodom Sihamouni in 2016 also showed him with rolled-up sleeves and an open collar, drawing unfavorable social media comments according to the Cambodian press.

Duterte, the oldest ever Filipino president, regularly displays similar unconventional behavior at official functions in the Philippines.

In 2006 he revealed that he used to take fentanyl, a powerful painkiller that is often prescribed for cancer pain and other chronic ailments, because of a spinal injury from previous motorcycle accidents.

Duterte has also said he suffers from daily migraines and ailments including a rare cardiovascular condition characterized by inflammation of blood vessels usually due to smoking.