They've been blamed for a host of symptoms — from insomnia to sexual problems.

But a new major study appears to provide evidence for another side effect: weight gain.

Harvard researchers studied prescription data from nearly 200,000 US adults and found that those who took the tablets escitalopram, paroxetine, and duloxetine gained up to three pounds after two years taking the drugs. 

Those on those medications were 10 to 15 percent more likely to gain at least five percent of their starting weight compared to users of five other commonly-prescribed antidepressants, like sertraline.

In comparison, other medications like duloxetine, or Cymbalta, was associated with just a pound of weight gain over a two year period. 

 All eight drugs studied were associated with some weight gain over a two year period.

However, the least amount of weight gain was seen in patients who took bupropion or Wellbutrin — one of three drugs studied that is not an SSRI — who were seen to add under a pound to their waistline. 

Only duloxetine and paroxetine were linked with no weight gain at all after six months.   

The researchers studied compared weight changes at 6, 12, and 24 months after starting one of eight common antidepressants: sertraline, citalopram, escitalopram, fluoxetine, paroxetine, bupropion, duloxetine, and venlafaxine.

All participants had to have not taken antidepressants in the six months prior to the study, although it was not possible to confirm if prescriptions for the drugs had been given before that. 

While anecdotal reports of weight gain are common, the FDA does not cite it as an official side effect of most SSRIs. 

Antidepressants are taken by one in 10 Americans, and are a highly effective tool to help lift the fog of depression as well as anxiety disorders.

 A study from earlier this year found that antidepressant prescriptions for young adults and teens have increased by nearly 64 percent since 2020.

Dr Joshua Petimar, a professor in the Department of Population Medicine at the Harvard Pilgrim Health Care Institute and co-author of the new study said of his findings: ‘Patients and their clinicians often have several options when starting an antidepressant for the first time. This study provides important real-world evidence regarding the amount of weight gain that should be expected after starting some of the most common antidepressants.

‘Clinicians and patients can use this information, among other factors, to help decide on the right choice for them.’

The study, published in Annals of Internal Medicine, did not feature a control group — whereby weight gain is compared against someone not taking the medicine.

This means the small increase in weight over a two year period could be due to chance, or the mental health problem itself.  

Dr Petimar and fellow researchers from Harvard Medical School and Harvard Pilgrim Health Care Institute analyzed prescription data from eight US health systems that are part of PCORnet, the National Patient-Centered Clinical Research Network.

They studied 183,118 adults aged 18 to 80 who had recently started taking one of eight common antidepressants. 

At six months of use, patients taking escitalopram, paroxetine, and duloxetine  gained approximately 0.66 to 0.88 pounds more than sertraline patients — and were 10 to 15 percent more likely to gain at least 5 percent of their starting weight.

Dr Jason Block, senior author and general internal medicine physician and professor at Harvard Medical School, said: ‘Although there are several reasons why patients and their clinicians might choose one antidepressant over another, weight gain is an important side effect that often leads to patients stopping their medication.

‘Our study found that some antidepressants, like bupropion, are associated with less weight gain than others. Patients and their clinicians could consider weight gain as one reason for choosing a medication that best fits their needs.’

Some antidepressants are thought to increase appetite, leading someone to take in more calories. 

Selective serotonin reuptake inhibitors are the most prescribed type of antidepressant. 

Common SSRIs include fluoxetine, citalopram, sertraline, paroxetine, and escitalopram.

Doctors also often prescribe Serotonin-norepinephrine reuptake inhibitors (SNRIs). Common SNRIs include venlafaxine and duloxetine.

Bupropion, or Wellbutrin, is a norepinephrine/dopamine-reuptake inhibitor (NDRI).

It is not fully understood how these medications work to control anxiety and improve mood, but experts believe it is due to the impact the drugs have on crucial hormones in the brain.  

SSRIs specifically increase the levels of serotonin in the brain by inhibiting the reuptake (absorption) of serotonin into brain cells. 

This makes more serotonin available to improve message transmission between neurons.

SNRIs, meanwhile, increase the levels of both serotonin and norepinephrine in the brain. They inhibit the reuptake of these two neurotransmitters, making them more available to enhance communication between neurons.

By increasing the levels of these neurotransmitters, anxiety is said to be reduced, mood is improved, and overall emotional well-being is enhanced.

Other recognized side effects of antidepressants include nausea, dizziness, restlessness and sexual dysfunction.

Some patients say that sexual problems continued for years after they stopped taking SSRIs, however large scale, high quality studies have yet to prove this conclusively.

Psychiatrists maintain that antidepressants are highly effective for treating mood disorders, with some studies suggesting the pills saved at least 30,000 Amercians from suicide between 1988 and 2002 alone.