The scene at the White House yesterday evoked the early days of the Covid pandemic: President Trump, standing at a lectern, offered Americans medical advice that contradicted scientific research.

This time, though, the subject was not hydroxychloroquine or injected bleach. It was a rise in autism diagnoses — and a purported connection to Tylenol and vaccines.

For years, scientists have studied a possible link between pregnant mothers’ use of acetaminophen, the active ingredient in Tylenol, and neurological conditions like autism and A.D.H.D. The findings are complex. Some studies suggest a link; others do not. None have found proof of a causal relationship.

Yet Trump spoke as if the connection were definitive. He instructed pregnant women to avoid the drug. “Don’t take Tylenol. Don’t take it. Fight like hell not to take it,” he said.

Trump also said that he and Robert F. Kennedy Jr., the health secretary, had long discussed the possibility that vaccines are linked to autism. “They pump so much stuff into babies, it’s a disgrace,” he said.

Dozens of studies over the last three decades have failed to find any link between vaccines and autism. Scientists say the idea has been debunked.