Well, VT, I guess making sure your GP starts taking it seriously is an obvious step, but one other thing you could try is taking all your prescriptions to the local chemist and asking him/her their opinion on the combination of medicines.

I remember when my first wife was in hospital and the resident chemist would come round every few days just to check what the doctors were prescribing. She said she had to keep an eye on them because they did, on occasion, prescribe conflicting drugs.