If it is prescribed by a doctor, then there is just the issue of smoking it. The elders could argue that if someone sees you smoking it, then there is the issue that it might stumble someone. Others may not know that you are doing it legally.
If you did it under a doctor's care and didn't flaunt it, I can't see how it would be a DF offense. However, if there was another way of taking it in (in a pill, salad or something) then there could be the argument that it is doing more harm to your body (lungs) than just taking a pill.
In the end, it depends on whether the elders are out to get you and/or you flaunt it.
Rub a Dub