@ddljohn-- I don't think that penicillin shots cause side effects unless the person is allergic to penicillin. There can be a small bruise at the injection site, but that's no big deal.
I had to get a penicillin shot for strep throat once. I didn't really want it but I was glad I got it afterward. It was sick for weeks and I recovered fairly quickly after the penicillin. I think that penicillin is kind of miraculous.