How rare is steven johnson syndrome i have to take antibiotics ive taken them lots of times before?

The potential of a life-threatening condition, such as Stevens-Johnson Syndrome, should not be part of decision making when needing an antibiotic as the infection risk is paramount. This is a rare condition which is unlikely to afflict the average patient who has prior experience with antibiotics. (Some other classes of medications are far more risky, such as lamotrigine.)

As rare as 1-5 in a million, even less likely if taking antibiotic that you have tolerated in the past.