Andres Manuel Lopez Obrador said his doctors had been concerned enough to administer a litre of rehydration fluids.
He said in a recorded chat from the National Palace in Mexico City — where he lives and is isolating — that doctors wanted to fly him back to the capital on a stretcher.
Mr Lopez Obrador was on a working tour of the Yucatan peninsula on Sunday when he tested positive for the coronavirus, his third bout of Covid.
But he wrote in his social media accounts on Sunday that “it isn’t serious”.
Reports in the local press that day said Mr Lopez Obrador felt faint on Sunday morning and had to cancel his tour, something his presidential spokesman denied at the time.
But on Wednesday, the president acknowledged it “had become complicated … I had a crisis because my blood pressure suddenly went down” during a meeting with military engineers working on his pet project, a tourist train on the Yucatan peninsula.
“It was like I fell asleep,” he said.
“I didn’t lose consciousness but I did briefly faint because of the low blood pressure.”
The president said he was flown back to Mexico City on an “air ambulance” but said he was not carried on a stretcher.
Mr Lopez Obrador, 69, who has acknowledged a history of heart problems, said his heart was “not at all affected”.
On Tuesday, US ambassador Ken Salazar, who frequently meets with Mr Lopez Obrador, said he too had tested positive for Covid.
He wrote on his Twitter account: “I am well.”
Mr Lopez Obrador caught Covid in early 2021 and was ill but recovered after receiving what he described at the time as an experimental treatment.
In January 2022, he announced he had come down with Covid a second time amid a spike in coronavirus infections in Mexico.
Mr Lopez Obrador declined to enact mandatory mask mandates and he refused to wear a mask even at the peak of the pandemic unless it was absolutely necessary, like on airline flights.
He famously refused to use Mexico’s presidential jet, which he recently announced had been sold to Tajikistan.
While Mr Lopez Obrador remains in isolation — he said he has spent the time working on speeches — interior secretary Adan Augusto Lopez has been filling in at the daily presidential morning news briefings.
That could provide a boost for the interior secretary’s flagging campaign to win the presidential nomination of Mr Lopez Obrador’s Morena party for the 2024 elections.
Mr Lopez, who is not related to the president, currently trails Mexico City mayor Claudia Sheinbaum in most polls of the primary race.