US President Donald Trump has claimed there is a link between the use of painkiller Tylenol by pregnant women and an increased risk of autism in some children.
Going against current scientific advice and medical opinion, he said the drug, known as paracetamol in many countries, "is no good" and women should "fight like hell" to only take it in extreme cases, such as for high fevers.
Medical bodies say the drug is safe and that it remains the best treatment for pain and fever during pregnancy.
Autism is a form of lifelong neurodivergence and disability that affects how people experience and interact with the world.
It is a spectrum, meaning it includes a wide range of characteristics which vary from person to person.
It can include those with high support needs who are non-speaking, and those with above-average intelligence who might struggle with social interaction or communication.
Some autistic people may have intense interests, prefer order and routine, and use repeated movements or actions to calm themselves or express joy.
Autism is not a learning disability or a mental health condition.
However, the UK National Autistic Society says around a third of autistic people also have a learning disability, and rates of mental health problems are higher.
There is no blood test or brain scan to determine who has autism. Instead, a diagnosis is based on a doctor's observations.
The number of children diagnosed with autism has risen sharply in many countries.
In the US, one in 31 children were identified with autism in 2022, up from 1 in 149 in 2000. The rate for eight-year-old boys was significantly higher than for girls.
In the UK, research published in the Lancet medical journal in 2023 found that one in 34 children aged 10 to 14 years old were diagnosed with autism (in 2018).
Comparing autism rates for different countries is difficult. The way the figures are collected tends to change based on diagnostic practices, awareness levels, healthcare access and cultural differences.
Research published in 2022 looked at 71 studies and found an average prevalence of around 1%. Numbers ranged from 0.075% in Bangladesh to 3.9% in Australia.
While the number of people officially identified or diagnosed with autism has risen sharply, most researchers believe that this reflects increased awareness and changes to the way it is classified.
For example, some people who have difficulty with social interaction and non-verbal communication were once told they had Asperger's Syndrome.
But, in 2013, the diagnostic criteria of ASD – or Autism Spectrum Disorder – was broadened to include people with Asperger's Syndrome in the US.
The same wider definition has been used in countries, including the UK, which saw an eight-fold rise in new autism diagnoses between 1998 and 2018.
A 2007 study in the UK found that as, long as the same criteria was being used, rates of autism were similar in adults and children. This supports the idea that it is more recognised today than in the past.
However, US Health Secretary Robert Kennedy Jr has sharply criticised that view, describing it as "epidemic denial".
At a news conference in April, he described autism as "preventable". He has previously offered several debunked theories about the causes, including blaming vaccines, despite a lack of evidence.
Acetaminophen, or paracetamol, is the recommended first-line medication for pain and fever during pregnancy by the American College of Obstetricians and Gynaecologists, the UK's Royal College of Obstetricians and Gynaecologists, and other medical organisations worldwide.
There is some evidence that high fevers during pregnancy, particularly during the first trimester, can harm the baby, with a higher risk of miscarriage, preterm birth, or birth defects called NTDs, or neural tube defects.
Following Trump's announcement, health officials stressed that paracetamol remains the safest painkiller available to pregnant women.
UK Health Secretary Wes Streeting said: "I trust doctors over President Trump, frankly, on this."
As with any pain-relieving medicine, it should be used at the lowest effective dose, for the shortest possible time and as infrequently as possible.
Ibuprofen is not usually recommended, unless it's prescribed by a doctor, especially if someone is more than 20 weeks pregnant, because it can affect the baby's circulation and kidney function.
Paracetamol is considered to be one of the safest drugs in the world when taken at the correct dose. Like all medicines, there is an overdose risk if more than the recommended dose is taken.
For adults, the usual dose is one or two 500mg tablets at a time, up to four times in 24 hours, according to the National Health Service (NHS) in the UK.
It describes paracetamol as the "first choice" pain killer for women who are breastfeeding.
US Health Secretary Robert F Kennedy Jnr also signalled that a drug called leucovorin, normally given to cancer patients to reduce the side-effects of chemotherapy, would soon be approved as a treatment for children with autism.
But researchers have cautioned that the science is still in the very early stages, and more work is needed before any firm conclusions can be reached.
Folate, also known as vitamin B9, is found in many foods and helps the body form health red blood cells.
The Autism Science Foundation says that low folate levels during early pregnancy have been linked in some studies to an increased risk of autism in children, though the findings are not consistent.
Studies in Norway, the US and Israel found that mothers who took folic acid supplements around conception had children with a 30-70% lower likelihood of autism. Other studies did not find a significant association.
The charity says the suggestion that folate may improve some symptoms comes from trials of a cancer drug known as leucovorin or folinic acid.
Some research has shown that unlike regular folic acid, folinic acid can more easily cross the blood brain barrier and address a deficiency of the vitamin.
Four small, randomised trials, all using different doses and measurements of success, have been carried out.
One, from 2016, studied 48 autistic children in the US and found improvements in verbal communication compared with a placebo.