The number of antidepressants prescribed to children in England, Scotland and Northern Ireland has risen over the past three years, figures obtained by BBC's File on 4 reveal.
In England, there was a 15% rise. Scotland saw a 10% increase. And in Northern Ireland the number rose by 6%.
In total, there were 950,000 prescriptions issued between April 2015 and March 2018.
Experts have linked the rise to waits for specialist mental health services.
Antidepressants should prescribed to children only under close supervision.
NHS England, NHS Scotland and the Health and Social Care Board in Northern Ireland all say they are committed to improving child mental health services.
NHS Wales was unable to provide prescription figures because it does not hold the data requested.
The figures were obtained by Freedom of Information requests and relate to a group of powerful antidepressants known as selective serotonin reuptake inhibitors (SSRIs).
The total number of prescriptions rose from 290,393 in 2015-16 to 330,616 in 2017-18.
The steepest increase was seen in the youngest patients, those aged 12 and under, where the number of prescriptions rose on average by 24%, from 14,500 to almost 18,000.
Dr Bernadka Dubicka, who chairs the child and adolescent faculty at the Royal College of Psychiatrists, said: "Currently only one in four children and young people are treated for their mental health problems.
"The fact that prescriptions for antidepressants are rising could reflect a slow but steady move towards treating everyone who is unwell.
"But the importance of giving children access to psychological therapies cannot be overstated.
"What we don't know from today's data is why these antidepressants are being prescribed, and how.
"It is vital that they are being used judiciously, monitored carefully, and the risks and benefits of taking them are assessed in each individual case."
The mental health charity Young Minds says waiting times for specialist child and adolescent mental health services (Camhs) have been increasing too, making it difficult for families to get support.
Marc Bush, chief policy advisor at Young Minds, said: "The government recently suggested that the average waiting time for general access to Camhs is around 12 weeks, which is a long time to be in distress.
"And that is why front-line professionals are turning to prescription pads, because they're thinking, 'I'm seeing someone in front of me in crisis with a level of distress I don't want to leave them with - how do I offer them some kind of alleviation from that?'"
In rare cases, antidepressants can trigger thoughts of suicide and self-harm in children.
And the National Institute for Health and Care Excellence (NICE) has clear guidelines:
- Prescriptions should be written by a child psychiatrist within Camhs, not GPs
- Antidepressants should be offered only concurrently with psychological therapies
- The patient must be monitored to check for adverse reactions
However, File on 4 has heard concerns these guidelines are not always followed.
Reece Burrowes, 17, was found dead in a park in south-east London on 6 December 2015, days after being prescribed an antidepressant called Sertraline by his GPs.
Although the inquest into Reece's death recorded a verdict of death by suicide and no blame was assigned to Reece's doctors or Sertraline, File on 4 has been told several of the NICE prescribing guidelines were not followed by doctors.
Reece's stepfather, Simon Banks, said: "There are NICE guidelines, which you know in theory they should follow, but I know one of the concerns is that their thinking, 'If I do refer them to Camhs, there's potentially a waiting list.'
"From what we've gone through, our view would be don't give the medication, and wait.
"But I can imagine it's a very difficult call for the doctor to make."
Dr Shruti Garg, a Camhs consultant at the Manchester Foundation NHS Trust, said problems with young people transitioning out of Camhs services could also be leading to increases in prescription rates.
"Camhs services across most parts of the country cover only up to the age of 16 - adult mental health services start at 18, so there is a gap for the provision of services across these services."
The Department of Health in England told the BBC it had pledged £1.7bn to transform Camhs.
This, it said, would help to provide greater access to a wider range of therapies, although there would still be patients who would benefit from antidepressants.
In Scotland, Minister for Mental Health Clare Haughey said young people's mental health was a priority and £5m has been invested to create a taskforce to improve Camhs services.
The Health and Social Care Board in Northern Ireland said the vast majority of referrals accepted into Camhs were seen within nine weeks and it was currently planning to spend an extra £1m on Camhs projects.
If you, or someone you know, have been affected by mental health issues, you can access resources online via BBC Action Line.
File on 4: Counting the Cost: Anti-depressant Use in Children is on BBC Radio 4 at 20:00 BST on 24 July.