Most people who are prejudiced against fat people (thinking them lazy, weak willed, and so on) would claim they are not. This is natural. People like to believe they are unprejudiced. If you say to them, in effect, "hey I know most fat people are lazy and weak willed, but not me, I was on a medication so this isn't my fault," you run the risk they pay more attention to the first half of the sentence than the second. They could consider you prejudiced on top of whatever negative things you're worried the weight might make them think.
You may deny the first half of the sentence, but if it wasn't true, why are you saying the sentence at all? Ah, because while you don't think that, you worry they think that. That makes the sentence "hey I know you think most fat people are lazy and weak willed, but not me, I was on a medication so this isn't my fault." Is that making it better? I would say it is not. Now you've accused them of being prejudiced. While you would never word the sentence either of these ways, however you word it, you risk them interpreting it with one of those two "first halves" and then you've got a bigger problem than what they think about overweight people.
To show you care about your appearance, take extra care with your clothes, accessories, shoes etc and be appropriately polished and disciplined. To show you work hard, present a resume that lists a lot of hard work, and answer questions the way hard-working people answer questions. To show you have self discipline, present a resume that is clearly the result of striving towards a goal, and answer questions the way disciplined people answer questions.
Discussing your health in an interview is generally a huge no-no. You can sneak it in by telling a story that illustrates something you want to illustrate, and includes it as a side mention that inspires. Say they ask how you deal with the stress of multiple deadlines or fast-changing requirements, or how you deal with a difficult team-mate, or a time you overcame a challenge. Sliding in "while I was beating a life-threatening illness last year" in the middle of the story should be easy enough to do. You might even be able to sneak in "I'm just so grateful to be alive and to have the only longterm consequence be this extra 80 pounds I gained from the meds and will likely always have" as a positively-focused part of that story. But that's a side note. Don't address it head on: it's not going to help and could hurt.