Sulfamethoxazole and suggestion

I have had a horrible reaction to Sulfameth. At first, it was effective in treating my acne, but on the 5th day, I began to get severe headaches, stomach pain (which I now realize was the pain caused by water retention), and a rash began to develop. I lowered to dose to 800 mg/day at the suggestion of my dermatologist, and now the rash covers my body and my face is bright red and itches like crazy (I want to reach for some sandpaper). I don't know why this drug is on the market, because I have heard from friends people often react to Sulfameth and similar drugs.