At my sixth form, we all initially had to take four AS levels, but after about the first week they told us we could drop one if we wanted. Apparently the only reason we are advised to take four is if we get a U in one of them then we can drop it and do the others at A level next year because Unis only look for three. I have now dropped to three and I have more free periods so more study time etc.
So yeah, you should be allowed to, especially for a medical reason.