US President Donald Trump says he is in "perfect" health, takes more aspirin than doctors recommend, uses makeup to cover bruising on his hands, and does not get regular exercise because he finds it "boring".

In a lengthy interview about his health, Trump also told The Wall Street Journal that he underwent a CT scan in October, after incorrectly telling reporters that he had undergone a more detailed MRI scan.

Trump, 79, is the oldest president inaugurated in US history, and signs of age have been showing. He has appeared to fall asleep in some meetings and not to be able to hear questions.

The Journal said Trump "expressed irritation about the public debate over his health".

"Let's talk about health again for the 25th time," he said at the start of an "impromptu phone call" as the newspaper prepared to publish a story about Trump's health.

Trump said he had been taking larger than recommended doses of aspirin for the last 25 years, even as he said it caused him to bruise easily.

"They say aspirin is good for thinning out the blood, and I don't want thick blood pouring through my heart," Trump said.

He takes 325mg of aspirin per day, rather than the 81mg dose sometimes recommended by doctors.

"They'd rather have me take the smaller one," Trump said.

But he does not want to switch routines after so many years. "I'm a little superstitious," he said.

The president also talked about the medical scan he received at Walter Reed National Medical Center, which he previously said was an MRI scan.

"It wasn't an MRI," Trump told the Journal. "It was less than that. It was a scan."

A doctor for Trump, Navy Capt Sean Barbaella, said in a statement provided to the Journal that Trump had undergone a CT scan, "to definitively rule out any cardiovascular issues" and that the testing revealed no abnormalities.

After Trump was diagnosed with chronic venous insufficiency, a common condition in older people, he was advised to wear compression socks but he declined.

"I didn't like them," Trump said, adding that he now gets up from his desk more regularly, which has improved swelling in his legs.

Still, the president said he preferred not to do any form of regular exercise, other than golfing.

"I just don't like it. It's boring," Trump said. "To walk on a treadmill or run on a treadmill for hours and hours like some people do, that's not for me."

The president maintained that he had not nodded off during public meetings, saying he closed his eyes at times because he finds it "relaxing". As for the bruising seen on his hands in some photos, Trump said he now carried makeup in case his hand got "whacked".

"I have makeup that's, you know, easy to put on, takes about 10 seconds," he said.

Trump said he did not have trouble with his hearing, and when photographers have taken photos of him when he appears to be sleeping, he said "sometimes they'll take a picture of me blinking".

"And they'll catch me with the blink."

Barbabella said in the statement to the Journal that Trump was in "exceptional health and perfectly suited to execute his duties as Commander in Chief".