I have been on tamoxifen for about 17 months and I have not gained any weight. There's no magic to it. Avoiding weight gain on tamoxifen involves the same old boring edicts - watch what you eat and exercise regularly. I aim to run or do a gym class at least 4 times a week.
Having said that I do find I have to constanly monitor my weight and work hard to keep it stable - maybe a little harder than before BC, but not much. I know some experts advise against weighing yourself everyday but I find a daily weigh-in helps keep me on track. If I don't weigh myself for a few days I seem to relax my eating habits and my weight inevitably creeps up.
The other thing that has helped me avoid weight gain is an app called My Fitness Pal. I use it to record everything I eat and the exercise I do to help ensure I don't accidentally consume way too many calories. I think it is incredibly easy to kid yourself that you are eating a healthy, moderate diet when in fact you are taking in way more calories than you need. This app has been a big reality check for me.
I think the other thing to keep in mind is that many people - both women and men - gain weight in middle age for a variety of reaons, not just us BC survivors on tamoxifen. Personally I doubt that the weight gain many woman complain about is due to the medication. It's just that our metabolisms are slowing with age, hormone levels are dropping, we are perhaps not as active as we used to be, we live in fast food culture etc etc etc. Unfortunately there's no super easy way to stay slim. Just gotta stay focussed and disciplined. Good luck, Janet.