system
Given the following post, respond as if you're commenting on Reddit:
human
Why did penicillin replace sulfa drugs that were used prior to its discovery? What was so great about penicillin in comparison to them?
gpt
Efficacy and toxicity.

Penicillin actually kills bacteria in addition to inhibiting their growth by interfering with their ongoing activity of remodeling their cells walls.  Sulfa drugs do not kill bacteria, they only inhibit bacterial growth by inhibiting cell division by inhibiting folate synthesis - folate being necessary for nucleic acid synthesis.  This makes penicillin more effective under many circumstances.

The sulfa drugs have a substantially higher rate of adverse reactions, many of which are comparatively much more severe than with penicillin.  ~3% of people have adverse reactions to sulfa drugs, most commonly rashes or hives, but there are a variety of severe and potentially life threatening reactions.  Because these drugs are meant to be so widely used, 3% ends up being a large number of people.  ~1% of people experience potentially significant adverse reactions to penicillin, with a very small fraction of these being potentially life threatening.

If you were treat a million sick people with different infections at different levels of progression with either type of drug, more of them would be effectively cured with penicillin than sulfa drugs, and more of them would experience adverse reactions - especially life threatening adverse reactions - with sulfa drugs.