He added: “I won’t say what, but a lot of people think something happens, but you can’t do that kind of stuff. You just can’t do that kind of stuff.”

Trump went on to say that he, too, would take a drug test ahead of the debates.

“I would take one too,” Trump said. “He’s taking something, or taking something that gives him some clarity … Maybe he had 15 cups of coffee.”

Trump continued, doubling down on Biden’s primary debate performance, while also taking a swipe at his running mate, Sen. Kamala Harris, D-Calif., who also launched a presidential bid this cycle.

“I’ve seen him in the debate with the other people, including Kamala, and nobody treated him worse than her, called him a racist … Kamala has her own problems, she dropped out before they got to Iowa. I think she’s not a good choice, compared to Mike Pence, who’s been a great vice president,” he said.

But Trump returned, again, to the topic of a drug test, calling it “very strange” and saying: “I think he should take a drug test.”

“Joe is lost, Joe is lost,” Trump said. “We can’t have a president that is mentally lost.”

Last month, the Biden campaign denied that the former vice president was on any drugs during the debates and blasted Trump as a liar.

“We know it’s unbelievable to Donald Trump, but Joe Biden can speak honestly without a shot of truth serum,” Andrew Bates, a spokesperson for Biden’s campaign, told Fox News. “We’d like to see Trump do the same.”

The early months of Biden’s third White House bid were marked by uneven debate performances and winding town halls in Iowa and New Hampshire. The floundering led to numerous stories about Biden’s gaffes and near-constant attacks from Trump about Biden’s mental fitness.

Since then, however, the former vice president has drawn praise for both his performance against Sanders in March and, more recently, for his speech at last week’s Democratic National Convention.

Fox News’ Andrew O’Reilly and The Associated Press contributed to this report.