gaining weight due to medicin

Dear Elah
The antidepresant that you have been taking-Remeron or in its generic name Mirtazapine might in some cases have an adverse effect of slight weight gain or edema yet it is important to not that after the weight gain of the 4 Kg you have a BMI of 20 which is the lower end of the normal BMI -which is between
20and 25 If you will lose again this weight the weight of 50 kg will give you a BMI of 18.5
This will put you in the underweight range

I understand that you changed the anti depressant to Efexor which will less likely
have this side effect but I think you should focus more on why you are so anxious regarding body shape and weight issues.
The best way to explore it is in psychotherapy which will help you to develop a less vulnerable self esteem