Except for people that are allergic to penicillin the only thingit does is kill bacteria. However penicillin (and otherantibiotics) does not distinguish between harmful disease causingbacteria and helpful symbiotic bacteria (e.g. intestinal bacteriathat help with digestion, harmless bacteria in the mouth that blockharmful bacteria or fungi from finding places to grow) which oftencauses you to get sick unless you replace the missing harmlessbacteria by taking probiotics as soon as you stop thepenicillin.People that are allergic to penicillin can have unpleasant oreven fatal reactions when they take penicillin, but this is whattheir own immune system does to them, not what the penicillin didto them directly.