Rapid Weight Loss!

I'm a smoker, and whenever I smoke, I go to the bathroom more. I'm very skinny, and I want to gain weight, and when I quit smoking for a week, I gained 2 kg because I eat more and go to the bathroom less. Now it's very hard for me to quit smoking because all of my friends smoke, and whenever they're around, I want to smoke as well.

This is a huge problem because now no matter how much I eat, I smoke after a meal, then go to the bathroom. This makes me lose weight, and I'm too skinny to lose any more weight.

Any advice? Like which foods will make me go to the bathroom less, how can I smoke less, etc?