You just never know. The SSRI's have a reputation for causing weight gain and sexual side effects and even when they are working for the orignial intend purpose, these side effects cause many to stop taking them. I was on two SSRI's for 5 yrs, one of them
Paxil which is known to be the worst offender for weight gain. I lost 40 lbs on the two SSRI's. There is really no way for anyone to know which way it will go until they try it. It they start gaining weight, they can stop it and try another that many not have that side effects. Everyone is different when it comes to side effects.
If you read the side effects of many medicines, you will notice many report insomnia as well as sleepiness and weight gain as well as weight loss, and diarrhea as well as constipation. On and on. It is trial and error and one can only know who they will be affected by trying them.