Autistic children as young as 18 months old are being injected with human stem cells derived from umbilical cords in unapproved, unproven and potentially harmful “treatments” that scientists warn are proliferating across the US under the active encouragement of the US health secretary, Robert F Kennedy Jr.

Clinics in Florida, Texas and other states are selling what they bill as “regenerative medicine” to families with autistic children who have intensive care needs. Parents who have taken their children through the process talked to the Guardian about their hopes and fears for a therapy that appears to be gaining ground in the US.

The procedure, which can involve the child being sedated with ketamine before receiving intravenous doses of millions of stem cells, costs up to $20,000 each treatment. Families are often advised to return for regular top-ups.

Profoundly stressed parents are being wooed to the clinics with promises that a high-dose infusion of umbilical cord stem cells can lead to dramatic improvements in their children’s ability to speak, socialise, or avoid aggressive or self-harming behaviour. Yet there is no scientific evidence that the procedure works – the most comprehensive clinical trial staged so far, a placebo experiment conducted by Duke University, found insignificant benefits for most of the 180 children tested.