Ans: It is not a very big surprise that penicillin has managed to save a lot of lives to be completely honest. However, there are some cases where penicillin didn’t do any good to the people for sure. There are some people who might have some sort of allergic reaction to penicillin. There are some who have rashes and itching as well as swelling in the skin due to the involvement of penicillin. These are some of the most notorious side effects of penicillin. Not to mention that some bacteria have become pretty much immune to this drug and hence it fails to have any effect on them.