I don't have any experience with persuading a child to stop breastfeeding really I'm afraid - we basically did child-led which meant I just barely missed getting my "breastfed a 4yo" medal :-) Sympathy, though - I remember enough about what my DS was like at 2 to be glad I didn't have to try stopping then.
What I do have is lots of experience with women thinking they have to stop because they need to take a medication when actually it's not necessary. Are you sure, in your case, that taking the medication is incompatible with feeding your DD? Have you looked it up in Hale, for a start? [I don't remember whether it's online these days but any bf community will have someone with a copy; if you need help PM me the details and I'll ask for you.] If you have and there really is a problem with this drug, another thing to consider would be how long it stays in the milk - e.g., if it's something you have to take once a day, would there be little enough of it in your milk by just before the next dose that nursing then would be safe? If so maybe you could keep that once a day nursing. If neither of these work, is there an alternative medicine that would have better characteristics? The main point is that most doctors have no clue about what's compatible with nursing, really, so many women get bad advice.
If none of that helps and you do have to stop, could you tell your DD the truth but not all of it? E.g. explain that you have to take the medicine, but if the reason is scary downplay it, and tell her the medicine is bad for children? (Point of advice: don't tell her it will make the milk yucky unless it really will, or unless you're positive you'd never let her find out otherwise, even if e.g. she broke her leg the day after you tell her this!)
