There have been 51 deaths among children and adults taking drugs for ADHD (attention deficit hyperactivity disorder) in the US since 1999. Yesterday the UK licensing authority, the Medicines and Healthcare Products Regulatory Agency (MHRA), said nine children had died in this country among a smaller population on medication. They declined to reveal the children's ages because of the possibility of identification.

In the US, doctors write 2m prescriptions for ADHD drugs for children every month and 1m for adults. In the UK, nobody knows how many people are on the drugs, which are licensed for children as young as six - although there are reports of them being given to children as young as three. A total of 361,832 prescriptions were written last year for Ritalin and other drugs of the methylphenidate class, which averages 30,153 a month.

The cause of death for two of the nine children who died in the UK was specifically heart-related: one had a heart attack and a second an enlarged heart. One was recorded as a "sudden death". One died of a haemorrhage in the brain and another of a swelling in the brain, two committed suicide and the last died of neonatal respiratory distress syndrome - presumably the mother was on the drugs.

The recommendation by an advisory panel of the Food and Drug Administration to put a "black box" warning - the most serious possible - on all ADHD drugs in the US is likely to be accepted. Pressure will mount now on the British authorities to warn publicly of the risk.

There is growing concern about the rising numbers of children being put on drugs. This class of drugs, known as methylphenidates, are amphetamine-based and it is thought they could cause heart problems in some children and adults because they raise blood pressure. There is already a warning on the drug most widely used in the US, Adderall, which is not licensed in the UK.

The FDA advisers said it was not certain the drugs contributed to the 51 deaths. "The data is only suggestive at this point, but because of the gravity of the side-effect, namely sudden death, physicians need to be made clearly aware of that concern," said Dr Peter Gross, the panel chairman and head of internal medicine at Hackensack University Medical Centre in New Jersey. One member of the panel was clear that he hoped the warning might slow down the soaring rate of prescription of the drugs to children who are inattentive or badly behaved at school. "I want to get people's hands to tremble a little bit before they write that [prescription]," said Steven Nissen, a panel member and cardiologist at the Cleveland Clinic.

Dave Woodhouse, psychologist and director of the Cactus Clinic, attached to Teesside University, which offers parents an alternative to drugs in the form of nutritional guidance and counselling, said: "One of the main issues is that fact that in the case of a lot of kids given Ritalin you don't know if they have a heart condition or not."

There are three methylphenidate drugs licensed in the UK - the bestselling Ritalin, Concerta and Equasym. The MHRA said there had been 521 reports of suspected adverse drug reactions, most of them in the past five years as more children have been put on them. It added that methylphenidate "is recognised to cause cardiovascular adverse effects", such as a racing or abnormal heartbeat and palpitations and increased blood pressure.