Not only is it unethical, it is ILLEGAL for the teacher to reveal to you that another child is on medication. This is a violation of HIPAA (which the schools are bound to follow).
In fact, under HIPAA, the only people at the school who are allowed to know ANYTHING regarding a child's medical conditions, etc.. are those that need to know. So for example, the only people that know my child has an IEP are his immediate teachers, principal/VP, counselor, nurse, and SPED teachers. Further, each teacher is only allowed to know just so much info that they need to deal with the child, not everything. The school has to go so far to protect which students in class have conditions and such (on medications, IEP's etc) from OTHER students, teacher helpers, parent volunteers, etc..
So, his music, strings, and art teacher do not even know he has an IEP because he does not need it for their classes. His PE teacher does know.
You should report it to the administration, because if she's done it to you she's done it multiple times and the administration needs to reinforce the privacy laws must be followed or the school could be held liable.
Now, that doesn't mean people don't know things. I spent an awful lot of time in my son's extended resource room and knew all the kids that were in there. I knew all of them had IEP's, because otherwise they wouldn't be in the room. We also could usually figure out what each child's disability was. My son was also pretty curious about why some kids got to use certain things and he didn't, but the teacher approached it more along the lines of what my son observed in class (you know you have seen x have trouble walking, my son would say yes, well this item is to help his feet get better at that) then giving away things they weren't supposed to. My son also knows what a few kids he has been in SPED with for a very long time are diagnosed with. I have not asked how he knows this, but I assume that the child has shared the information with mine OR if the information came from someone else, they had permission to share it with my son. Some of the parents of a few kids who are mainstreamed, but have some pretty severe disabilities in my kids classes have chosen to discuss the issues with the classroom so that the children understand more and accept the child more.
A better way of the teacher passing along the same information to you would have been for her to say something like that in her past experience teaching ADHD that it may take several medical trials, adjustments, etc.. until you have found the right fit for a child (which is a very true statement)