MANILA (AFP) - The son and namesake of the Philippines' former dictator Ferdinand Marcos said on Tuesday (Nov 23) that he took a cocaine test after President Rodrigo Duterte accused an unnamed presidential candidate of using the illegal drug.

Marcos Jr, who is among the frontrunners seeking to succeed Duterte in 2022, said in a statement he was tested on Monday and had submitted the result to three police agencies.

He told a local broadcaster he had tested negative.

"I really don't feel that I am the one being alluded to," said Marcos Jr, popularly known as "Bongbong".

"In spite of that, I believe it is my inherent duty as an aspiring public official to assure my fellow Filipinos that I am against illegal drugs. This is why I took a cocaine test yesterday."

Duterte, who has waged a deadly war on drugs that has killed thousands of people, has not identified the candidate he claims uses cocaine.

At times, he has referred to them using male pronouns.

"There's a candidate who was using cocaine - you can ask the rich people," Duterte said on Monday.

"Why did I not arrest that person? You don't know the rich, they go on a yacht or in the air, that's where they snort."

A leaked survey conducted in October by respected polling outfit Social Weather Stations reportedly showed Marcos Jr as the top preference for president with 47 per cent of voter support.

He was followed by incumbent vice-president and leading opposition candidate Leni Robredo (18 per cent) and celebrity mayor Francisco Domagoso (13 per cent).

Retired boxing champion and senator Manny Pacquiao was fourth with 9 per cent, followed by Senator Panfilo Lacson with 5 per cent.

Lacson said on Tuesday that he and his vice-presidential running mate, Senator Vicente Sotto, had voluntarily undergone "multi-drug testing" on Monday. He said they had "passed".

Pacquiao, who has admitted using marijuana and crystal meth in his youth, said he supported mandatory drug testing for those seeking a government post.

"Although I'm not in a position to judge anyone on this issue, I am willing to undergo a drug test anytime and anywhere," Pacquiao said on Friday, suggesting a hair follicle test offered "better results".

Domagoso on Friday also denied using drugs.

Duterte has been an ally of the controversial Marcos family, which had gone into exile in the United States after the patriarch's humiliating downfall in 1986.

But recently Duterte has been publicly critical of Marcos Jr, describing him as a "weak leader... saddled with baggage".

Duterte's daughter Sara had been widely expected to run for president, and potentially protect him from criminal charges in the Philippines and International Criminal Court prosecutors over his deadly drug war.

But she has filed her candidacy for vice-president, a position which holds very little power, and formed an alliance with Marcos Jr.

In the Philippines, the president and vice-president are elected separately.

Sara Duterte-Carpio told a rally of supporters on Sunday "we need to protect our candidate. Let us protect BBM ('Bongbong' Marcos)".

Duterte's chosen successor is now his close aide Senator Christopher Go, who made a last-minute entry into the race on Nov 13.

Many analysts consider Go unlikely to win.