I took both tamoxifen for 5 years and anastrazole for 10. I never gained weight until a few years later when I was in a horrible accident, so I don’t think this will be universal.
There are a ton of variables in people, topping the list is genetics and daily life.
Then there is other medical conditions, jobs, family obligations etc, etc.
Are you concerned about weight gain, or gained weight on one of these drugs?

I assume you want to avoid weight gain rather than trying to add weight? I've been on letrozole for 10 months and have lost a few pounds (normal weight in any case) but that seems totally unrelated to meds--just a bit more exercise and less dessert. Do AI and Tamoxifen actually cause spontaneous weight gain, or maybe just occur at the same time? Hope you get the answers to decide what works for you.

I am about to go on Arimidex. I am borderline obese and have concerns about whether I can lose weight on this drug. I'd appreciate any feedback on your experience. Thanks.