❌ This medicine is not good for my stomach.
✅ This medicine upsets my stomach / This medicine doesn't agree with me.
'위장에 좋지 않다'는 단순한 비교가 아니라 약 때문에 속이 불편하다는 뜻이라서, upset이나 doesn't agree with가 더 정확해요.
잘못된 예: I had to stop taking the painkillers because this medicine is not good for my stomach.
올바른 예: I had to stop taking the painkillers because this medicine upsets my stomach.