One in four Brits on anti-depressants in England have been taking them for more than five years.
Around eight million NHS patients are currently prescribed medication such as sertraline and citalopram, figures have shown, two million of which have been on the drugs long-term.
The drugs are used to help people with depression, anxiety and Obsessive Compulsive Disorder (OCD), with the number of prescriptions given out up by a million in just a year.
Doctors say many people taking SSRIs (Selective Serotonin Re-Uptake Inhibitors) continue to do so due to withdrawal symptoms, guidance for which was updated in 2019.
Figures obtained by BBC Panorama show a quarter of those on anti-depressants have been taking them for longer than five years.
The data however does not show how many of those are on the drugs long term because of prolonged mental health struggles, as opposed to avoiding coming off them.
SSRIs are one of the most prescribed drugs in the UK, according to scientists at the University of Oxford, with 2019 research showing more than £66m was spent by the NHS on them.
Prof Wendy Burn, a former president of the Royal College of Psychiatrists, said people were staying on them longer despite no precedent for any long-term impacts.
"Throughout my long and extensive career, I have seen people benefit from antidepressants.
"I see them working in my clinical practice, I see lives being changed by them."
She added: "People are staying on antidepressants longer, and we don't really have long-term studies that support that."
Oxford studies show SSRIs can benefit people's mental health in the short-term, while research at the University of Bristol has previously linked their use to conditions such as heart disease.
But evidence shows drugs manufacturers have long been aware of the withdrawal effects of the drugs, which occurs when a person's brain becomes accustomed to the medication before it is taken away without time to adjust.
Patients going cold turkey on SSRIs have previously reported severe mood swings, major depressive episodes and heightened anxiety.
Panorama reveals a a confidential 1996 memo from Pfizer, the original manufacturers of sertraline, discourages employees from volunteering the withdrawal symptoms.
"We should not volunteer to describe the withdrawal symptoms, but have an agreed list prepared in case they insist," the memo says.
A spokesperson for Pfizer - who no longer produces the drug - said all negative symptoms were reported "in line with its legal and regulatory obligations and updated sertraline labelling as required".
Data released by the Office of National Statistics (ONS) last month revealed a record number of Brits were taking time off work sick - with a rise in mental health problems blamed for the figures.
More than six million sick days were taken by NHS workers last year due to poor mental health.
The Royal College of Psychiatrists meanwhile says nearly a quarter (23 per cent) of adults with mental illnesses have had to wait more than 12 weeks for treatment, with many turning to A&E for help in a crisis.
For anyone who is struggling, Samaritans can be contacted 24 hours a day, seven days a week, on 116 123.
