If your medication causes sodium retention you will gain weight and may become obese

There are many medications that cause sodium retention/fluid retention/water retention/weight gain/obesity/edema/swelling. Here are some of them: steroids and HRT, most antidepressants, antipsychotics, anti-epileptics, anticonvulsants, and more.

If you have become overweight/obese because of drug-induced sodium retention, you need to reduce your salt intake in order to lose weight. If you do not reduce your salt intake you will continue to gain weight, regardless of how many or how few calories you eat.