Also, were you overweight to start with, in other words, could you stand to lose some weight given you don't exercise? But Gym is right, if the medication is causing this, you need to consult your doc to see if he has suggestions, if this is common, and if there's an alternative. For the most part, you gain weight by increasing food consumption. But just because you work, you still need to learn to fit into your schedule healthy meals or down the road you will suffer for it. Buying off the shelf is okay once in a awhile, but highly processed foods are not great to rely on. There are many ways to have a good meal waiting for you -- a slow cooker, for example, can cook beans or whole grains while you're doing other things or even sleeping. Vegetables are healthiest when cooked the quickest and simplest. Fish takes very little time to cook. It really doesn't take a long time to prepare and consume a real meal if you don't need to get fancy.
What medication are you taking and why are you taking it?