From personal experience I've been on mirtz for six years, started at 15 then to 30 then 45 for years I was advised to take just 15 because of trouble sleeping but I dot think it made a difference however overall the mirtz can help you sleep better.
I definitely think it helps.
Negative sides are most definitely weight gain, I always thought weight gain was due to increased appetite but mirtazapine made me realize this is not the case.... That pill be it 15 or45 adds at least 20lbs to my frame. Nobody has any reliable answer as to what it's down too, hormones etc.... But I know most people complain too. I would go as far as to even say it can add 40+ lbs and I'm sure people will agree with me.
On paper I read its good for mental nausea which I didn't notice a huge difference anything worth mentioning so probably placebo.
I believe the weight gain thing is quite complex.
SSRIs modify your hormone levels, it's what they are actually designed to do.
This can disrupt your endocrine system and have undesirable effects, possibly changing your ghrelin and leptin levels too, which mediate hunger/satiety (I can never remember which does which).
If you eat more, especially carbs, then you gain more weight.
However, SSRIs like Mirtazapine may also alter your insulin levels and increase your insulin resistance, which will also serve to further metabolic chaos and promote possible weight gain, as insulin is responsible for turning excess carbs into lipids which are then laid down as visceral fat. And carbs are the first things people turn to fort comfort eating, as well as getting little exercise to help mediate calorific use.
So yes, it's complex.
People on SSRIs often suffer from weight gain, but is the drug a causal factor? It's impossible to say and may even be a bit of a chicken and egg situation. I've had an endocrinologist say that SSRIs do cause metabolic issues and weight gain, but I've not seen any convincing research to prove it.
However, anything that has such a profound potential effect on your seretonin levels is almost certainly going to have an effect on other hormone levels too.
If it didn't, taking the drug wouldn't cause any 'side effects' whatsoever.