Eli5: How do certain medications cause weight gain? Don’t you need to be in a caloric surplus to gain weight?
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The “calories in, carolies out” paradigm of fat gain/loss is essentially true, but oversimplified and not especially useful without perfect knowledge and control of those those things. Medicines can make you tired and therefore move around less. They can reduce the calories you burn in the background. They can make you more hungry so you eat more – or if you resist the hunger, you body will likely slow down (a big problem for dieters, regardless of medication).
Medicines can also increase or decrease the amount of water stored in your body, which changes your weight without relation to calories.
Certain medications can mess with your feelings of hunger, or with how your body processes calories. So while yes, technically the medication isn’t causing you to gain more weight directly, the effects it has on the body can lead people to eat more than they normally would.
It doesn’t magically make weight appear out of nowhere. It makes you eat more, which makes you gain more weight.
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