Mr Trump made the accusation in an interview with Fox News host Jeanine Pirro, an excerpt of which was released ahead of its scheduled Saturday night broadcast.
“I think there’s probably — possibly — drugs involved. That’s what I hear. I mean, there’s possibly drugs. I don’t know how you can go from being so bad where you can’t even get out a sentence,” he said.
"I mean, you saw some of those debates with the large number of people on the stage," Mr Trump added. "He was — I mean, I used to say, 'How is it possible that he can even go forward?'"
It is the second time in the space of a month that the president has made the accusation, without providing any evidence. In an interview with the Washington Examiner at the Oval Office last month, Mr Trump called for both candidates to take a drug test ahead of presidential debates between the two.
"Frankly, his best performance was against Bernie. We're going to call for a drug test, by the way, because his best performance was against Bernie,” he said in the interview, published on 26 August.
“It wasn't that he was Winston Churchill because he wasn't, but it was a normal, boring debate. You know, nothing amazing happened. And we are going to call for a drug test because there's no way — you can't do that."
He added: “If you go back and watch some of those numerous debates, he was so bad. He wasn't even coherent. And against Bernie, he was. And we're calling for a drug test.”
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