US President Donald Trump has vehemently insisted that women who are expecting should "tough it out" and avoid Tylenol over an unproven link to autism and urged major changes to the standard vaccines administered to babies.

The Republican leader's announcement, rife with sweeping yet unsubstantiated advice, came as the White House has vowed to revolutionise health in the United States and as experts across medicine and science voice broad concern over the administration's initiatives that appear intent on unravelling decades of medical consensus.

Medical groups including The American College of Obstetricians and Gynecologists have long cited acetaminophen – the primary ingredient in Tylenol – as among the safest painkillers to take during pregnancy.

But Trump, who hammered on his message in increasingly emphatic terms, insisted that "taking Tylenol is not good" and to "fight like hell not to take it".

He said those who are pregnant should "tough it out" and that only an "extremely high fever" would justify taking the over-the-counter medicine. That's not true: fever and pain can pose serious threats to both the mother and the developing foetus.

Arthur Caplan, the head of New York University's medical ethics division, called Trump's claims "dangerous," "unscientific" and "full of misinformation".

"I worry that pregnant women are going to feel guilty if they took Tylenol. They're going to feel they let down their babies. They're going to feel that they were unethical in terms of trying to treat fever. That's just not fair, and it's not anything that anybody should be feeling," Caplan said.

Identifying the root of autism – a complex condition connected to brain development that many experts believe occurs for predominantly genetic reasons – has been a pet cause of Trump's health chief, Robert F Kennedy Jr. Kennedy for decades has spread debunked claims that vaccines cause autism.

On Monday he touted the drug leucovorin, a form of vitamin B first used to alleviate chemotherapy side effects, as an "exciting therapy" that could help children with the disorder whose symptoms vary widely across a spectrum. The FDA on Monday said it was approving the drug's tablet form to help a subset of children who have "cerebral folate deficiency."

Vaccines were also on the rambling agenda of Trump's conference. He ardently repeated anti-vax movement talking points as top figures in the administration, including Kennedy, nodded along.

He sowed doubt over standard vaccines including the MMR shot – which covers measles, mumps and rubella – and implied he would end the common use of aluminium in vaccines, the safety of which has been widely studied.

And Trump pushed for a major change to the routine vaccine schedule given to infants, insisting without evidence that there's "no reason" to vaccinate newborns against the incurable, highly contagious Hepatitis B.

That statement stands in direct contradiction of broad medical consensus formed over decades. Many experts say the best way to prevent maternal transmission of the disease, which can cause liver damage and cancer, is to vaccinate babies within the first day of life.

Trump's push comes days after an influential advisory panel handpicked by Kennedy stopped short of advising to delay the first dose of Hepatitis B vaccine by one month. They deemed more discussion was necessary – offering temporary relief to many experts across public health who said postponing that shot could have dire results.

"Spacing out or delaying vaccines means children will not have immunity against these diseases at times when they are most at risk," Susan Kressly, president of the American Academy of Pediatrics, said on Monday. "Any effort to misrepresent sound, strong science poses a threat to the health of children." (AFP)