Question

Why is penicillin completely useless for treating fungal infections? Why do antifungal agents cause severe side effects compared to penicillin?

Answer 1

Bacteria - Prokaryote
Fungus - Eukaryote
Humans - Eukaryote
Now that we've established that, antibiotics does not target any human tissue, it has it's function on inhibiting process or breaking down wall of bacteria, depending on the bacteria aswell as the antibiotics.
Antifungal agents has it's target in Eukaryote cells therefore are more prominenet to display side effects on your own Eukaryote tissue.

Questions?

Answer 2

thats clear enough thank you

Answer 3

Bottom line Antibiotics are only for bacterium. Though that was a great answer exuras