Attention-deficit hyperactivity disorder (ADHD) drugs alter the structure of children’s brains, scientists have discovered as they warn doctors against issuing prescriptions unless strictly necessary.

Scans of children with ADHD taking methylphenidate, best known as Ritalin, showed significant changes in the distribution of white matter, which is important for learning and coordinating communication between regions of the brain.

The difference between these children and other ADHD sufferers given a placebo was apparent after just four months.

A similar trial on adults showed no white matter changes between the methylphenidate and placebo participants, suggesting the brain is vulnerable to structural change while developing.

The study’s authors warned that the long-term consequences of Ritalin on the brain are unknown, saying the medication should only be given to children who are significantly affected by ADHD.

In 2017-18 75,000 children between the ages of six and 17 in England received a prescription for ADHD medication, according to the NHS.

This amounts to just over 1.5 per cent of boys and roughly 0.35 per cent of all girls of those ages.

The study also follows growing concern in recent years that university students - whose brains are still developing - are illegally acquiring the prescription-only drugs to improve their concentration.