Hi Angela,
It's true that most of the comments here are about the problems with Mirtazapine but actually it does work for insomnia.
The downside is that it can make you put on loads of weight, which is good for you as you say you were underweight, but for many it is a negative side effect.
The drug not only makes you crave food no matter how full you are already, it also changes the way your body sinthesises the food making your body produce excess fat stores.
When it makes you feel better because you can sleep you think -- so what if I've put on weight, I feel well--
In my case, the depressive symptoms were recurrent over many years.
GPs, who are not psychiatrists, just bump up the dose and send you on your way. A little relief then the depression comes back, and the weight piles on, the dose is increased, because GPs don’t know what else to do. Again, you feel better so you ignore the weight gain.
Finally I found myself clinically obese, going to the gym 3x a week, exercising really hard and putting weight on. Added to that I was experiencing long periods of extreme anxiety which I learned was common when on higher doses of Mirtazapine. Mind you I was still sleeping!
So I searched 'Mirtazapine, weight gain and anxiety'. It was this search that came up with this site. And what an epiphany that was. All of a sudden I realised that I was just a greedy, fat, anxiety ridden depressive. A lot of things started to make sense.
There's an awful lot you don't get told when you're put on medication, so much is left to the individual to find out and sometimes purely by choosing the right words for a search.
I'm a registered nurse so you'd think I would have access to all the nitty gritty of information, but I had to trawl the Internet to learn about Mirtazapine. The British National Formulary (BNF) and other drug info sites tell you very little about the side effects, but they are where GPs get their information from.
I haven't really answered your question but I hope I've enabled you to make more of an informed choice.
I'm sorry if I've reinforced the negative but I wish someone had told me all this 15 yrs ago.
Good luck xx