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In around a quarter of cases, long-term use of antidepressants is associated with a weight gain of 10 lb or more. For those who are already feeling less than stellar, this can be demoralising, fuelling anxieties about their health or appearance.

As Dr Derek Tracy of the Royal College of Psychiatrists explains, it is very normal to be concerned about weight gain.

"Few people will refuse to take medication because of this, but it will sometimes influence conversations about which particular medication to take," he says. "Doctors should be responsive to these concerns, and mindful of potentially replacing one problem with another one. This is particularly important for some people - for example, if there is a history of داء السكري من النوع 2 أو أمراض القلب."

Why weight gain happens

While gaining weight on antidepressants is far from inevitable (it all depends on how your body responds to that particular medication), it would be remiss to ignore the possibility.

In one recent large دراسة, involving nearly 300,000 people, people who had used an antidepressant for more than a year were found to be at higher risk of weight gain. Compared to the control group, they were 21% more likely to put on weight, although there was a lot of variability in which drugs had which effect.

Studies of this kind can't tell us why the weight gain happened, or even prove a causal relationship between antidepressants and weight gain. However, there are several theories as to why it might occur.

"Most antidepressants are effective through modulating the brain chemicals serotonin or noradrenaline; however, they can also unintentionally bind with other biochemical pathways in the brain and body, causing side effects," says Tracy. "We think that when weight gain occurs, some of this is due to binding with the 'histamine' system in the brain."

The older types of antidepressants (tricyclics) and the medication mirtazapine appear more likely to cause weight gain for this reason. Other types of antidepressants may have different effects on appetite and metabolism. And there are probably genetic factors determining your individual propensity towards weight gain.

In some cases, there is a simpler explanation - you lost your appetite while depressed, and have regained it now you're feeling better. This could be a positive thing, especially if you were underweight to begin with.

"Some weight gain can actually be an indirect sign of some recovery, if their mood is lifting and their appetite is returning to previous levels," says Tracy.

Alternatively, your depression might be associated with overeating or low energy levels. These symptoms might persist for a while even once you've started antidepressants.

"People vary, and while it is more common for people to lose their appetite and eat less when depressed, we recognise that there is a group of people who will eat more at this time. This is sometimes worsened by a lack of activity and exercise in depression," says Tracy.

Most people, however, will not pile on the pounds on antidepressants. Unlike antipsychotics, which do carry a substantial risk of weight gain, antidepressants typically only have a modest impact. On balance, you might think that it's worth gaining a few pounds for the sake of feeling better.

If your weight does seem to be spiralling upwards, solving the problem might be as simple as switching up your medication. For instance, venlafaxine, duloxetine and sertraline may be less likely to lead to weight gain than some other types of antidepressants. And while bupropion (also known as Wellbutrin) is not commonly prescribed in the UK, it is actually associated with modest weight loss.

"Anyone with concerns about weight gain should absolutely raise them with their doctor, and discuss if there might be more suitable specific medications for them and their physical and mental health," says Tracy.

This is particularly important if you have a history of اضطرابات الأكل or are already overweight. Antidepressants are not a one-size-fits-all medication, and it's worth persisting until you find something that suits your individual needs.

Of course, even where there is some weight gain, it's important to offset this against the benefits. Depression can be a debilitating condition and, if you're suffering, you deserve to receive proper treatment.

On top of that, if you've been struggling to get out of bed, antidepressants may well give you the boost you need to start exercising and eating more healthily. Chances are, this will improve your mood still further and help bring your weight under control, creating a kind of virtuous circle.

"We must remember the cost of not treating depression itself," says Tracy. "This can be worse for the body as well as the mind, including making it more difficult to exercise, be active and maintain a healthy lifestyle."