Folic acid, or folate, is a B vitamin that curbs the side effects of methotrexate. You'll find it in leafy vegetables, beans, and whole grains. You can also take it as a supplement.
Also, make sure you get enough calcium and vitamin D, especially if you take steroids (such as prednisone), which can cause bone loss.
Load up on omega-3 fatty acids, which are in fish like salmon and tuna. These good fats can help your body fight inflammation. Choose fish twice a week, or take a fish oil supplement.