Philippine President Rodrigo Duterte's admission that he used a powerful painkiller has prompted concern about his health, with lawmakers urging him Sunday to undergo a medical examination and disclose the results.

Duterte on Monday revealed that he used to take fentanyl, often prescribed for cancer pain and other chronic ailments, because of a spinal injury from previous motorcycle accidents.

He however said his doctor made him stop using it on learning he was "abusing the drug" by using more than the prescribed patches.

The firebrand leader has attracted controversy over his war against suspected users of illegal drugs, which has claimed thousands of lives, and his incendiary language against the United States and the United Nations.

Lawmakers said Duterte's remarks revived speculation about his health, including rumours during the election campaign that he suffered from cancer -- a claim Duterte repeatedly denied.

"To end this speculation, it would be better if his physician explains how the president manages the pain that he suffers," Duterte ally congressman Carlos Zarate told AFP.

Zarate added that a medical bulletin would clarify the state of Duterte's health, as fentanyl became controversial after pop legend Prince died of an accidental overdose of the drug in April.

Fentanyl, highly potent and addictive, is estimated to be up to 100 times stronger than morphine.

An outspoken Duterte critic, Senator Leila de Lima, supported Zarate's call.

"It is not just the illness itself that we should be worried about, but also the impact or side effects that the medications he is taking may have, especially on his lucidity and ability to make decisions with a clear mind."

At 71, Duterte is the oldest president of the Philippines.

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He has said he suffers from daily migraine and ailments including Buerger's disease, a cardiovascular illness characterised by inflammation of blood vessels usually due to smoking.

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Duterte cited ill health as the reason for skipping events during summits abroad. In Cambodia last week he said he might not even finish his six-year term.

Another critic, Senator Antonio Trillanes, told AFP Duterte's admission that he took more than the prescribed fentanyl dosage showed he "qualified as a drug addict".

However Duterte on Saturday denied any addiction.

"When there's regularity, my friend, when you take it and when there's a monkey on your back, that's addiction," he told a BBC reporter.

Doctors said fentanyl was regulated in the Philippines, with physicians needing a licence from the drug agency to prescribe it.

"The ones using (fentanyl) are usually people with harrowing pain or terminal diseases. Doctor monitoring manages risks of addiction," said Leo Olarte, former president of the Philippine Medical Association.

"A medical bulletin is good so the public won't be rattled."