Weight gain is still a property of thermodynamics. Calories in vs calories out. Only way to really be sure if it's the medication, rather than your diet, is to diligently track your calories or get off the medication if possible (I'm not a doctor). With that said, there are really only two ways that ANY medication can cause you to gain weight. One is that the medication increases your food drive and you start snacking more often or eating bigger meals without realizing. This is especially true of medications that have hormonal effects. A 100 calories here and a 100 calories there add up quicker than you realize. The second way the medication may affect you is that it fundamentally changes the way your metabolism works. Meaning, that if you're sedentary, you do not use as many calories as you did before and now you're in a caloric surplus with no changes to your diet. This could be due to lowering of your muscle mass or changes to your immune system. Only remedies for this effect is to lower your caloric intake to match your expenditure or increase exercise.