Donald Trump has called for himself and Democratic challenger Joe Biden to undergo a drug test before their first debate next month.
The president suggested his rival might be on drugs, saying he had noticed a sudden improvement in Mr Biden's performance in the Democratic TV debates.
Speaking to the Washington Examiner, Mr Trump offered no evidence but said: "I'm pretty good at this stuff."
Mr Biden is set to face the US leader in three debates before the election on November 3.
Mr Trump made a similar move in 2016, when he suggested his then-Democratic rival Hillary Clinton had been "getting pumped up" before their debates.
He challenged her to take a drug test ahead of their final TV debate.
On Wednesday the president made a similar claim, saying Mr Biden's debating had improved in the final debate.
He said Mr Biden "wasn't even coherent" during some of the debates he competed in during the Democratic primary season.
Mr Trump said: "I don't know how he [Mr Biden] could have been so incompetent in his debate performances and then all of a sudden be OK against Bernie."
