HI, thanks for using healthcare magic Weight gain is not a documented side effect of methotrexate though some persons may complain of weight gain with its use. It is possible that there can be there may interaction between the methotrexate and other medications that you may be using or the weight may be related to these other meds or lifestyle issues. It can be difficult to lose weight, it would involve not just using less but changing what is used. There are different types of diets but the two main ones would be low fat/low calorie and low carbohydrate. You can consider visiting a nutritionist or weight loss doctor for advice. I hope this helps