Medical authorities are mystified and concerned at figures suggesting antidepressant drugs are being prescribed for children, some less than a year old.

Records of the national drug buying agency Pharmac suggest thousands of prescriptions a year are being written for children under 10.

Antidepressants are powerful psychiatric drugs with potentially severe side-effects.

They are not usually prescribed to children younger than 8, and more commonly are not used on those younger than 13.

Depression is not found in babies and doctors the Herald spoke to could see no reason for prescribing antidepressants for them.

The Government's drugs regulator, Medsafe, warned in 2004 that antidepressants could increase the risk of suicide.

For children and adolescents, it said, the risk of suicidal ideas and behaviour from newer antidepressants called selective serotonin re-uptake inhibitors (SSRIs) generally outweighed their benefits.

Older "tricyclic" antidepressants were not generally recommended for those under 18 because of the risk of heart problems.

The number of state-funded antidepressant prescriptions has nearly doubled since 2000 to more than a million a year, costing the Government about $30 million.

Figures given by Pharmac to the United Future Party show 4728 antidepressant prescriptions were written for children under 10 in 2004-05, declining to 2425 in the last June year.

Causing most alarm are the figures for babies, even though they dropped sharply during the three years.

For 1-year-olds, 768 prescriptions were written in 2004-05, down to 24 by last year. For those under 1, there were 453 prescriptions in 2004-05 but only nine last year.

The numbers also declined for every other age group under 10, but each group remained in the hundreds last year.

Antidepressants are used for conditions including severe depression, anxiety disorders, obsessive-compulsive disorder and bulimia nervosa.

The tricyclic type were previously used to control bed-wetting, but this could not explain the prescriptions for babies.

Commonly reported side-effects of the SSRIs fluoxetine and paroxetine include diarrhoea, insomnia, sleepiness and tremors.

Pharmac yesterday said that the figures were correct. But it was mystified by them.

"I can't understand them," said medical director Dr Peter Moodie, who had not studied the figures until the Herald contacted him.

He said wrong coding of dates of birth could explain the single-digit figures, "but when it's hundreds, one assumes the figures are right".

Pharmac would re-check them and look to see which doctors had prescribed them to children.

Dr Moodie said he would also ask Medsafe to look at the issue.

Medsafe did not return Herald calls last night.

United Future health spokeswoman Judy Turner said the figures were frightening.

"Babies are born with only 15 per cent of their brain fully developed; 85 per cent of development happens from 0 to 3 years," she said.

"Surely the antidepressants will interfere with the hard-wiring of the children's brains and influence their long-term wellbeing."

The clinical director of paediatrics at Kidz First children's hospital in Otahuhu, Dr Wendy Walker, had never used antidepressants with babies, nor heard of anyone else doing so.

"I would never prescribe them in my practice as a hospital-based acute paediatrician."

Emeritus Professor John Werry, a child psychiatrist, said that "as far as we know" giving SSRIs to babies would not harm them.

"But one doesn't like to give growing and developing kids medications that affect basic bio-cyclic processes because it just doesn't seem like a good idea unless the kid is really in severe difficulty."

It would be rare in NZ to prescribe antidepressants before the teenage years, Professor Werry said.

The youngest child he had prescribed one to was a boy with autism aged 8 or 9, but that was "very exceptional".

Auckland City Hospital neonatal paediatrician Dr Simon Rowley said he would "never dream" of prescribing antidepressants to anyone under about 10. He was sceptical of the Pharmac figures.