text: do anti-biotics become less effective if you have taken them before? do you get immune to them? what happens is that bacteria, which the antibiotics attack, become immune to the antibiotics. over-prescribing anti-biotics is bad because it causes bacteria to develop resistance. this is why in the past very small quantities of penicillin-type drugs were necessary to treat a bacterial infection, whereas the dosages are now 100s of times greater. this is also why scientists are always struggling to keep up with bacteria which obtain resistance quickly.  the best strategy is to use antibiotics only when you are sure you need them. if you have a viral illness such as the flu, antibiotics are useless. also, if you take way too many antibiotics, you risk fungal infections. the reason is because fungi and bacteria are always in competition, and when you eliminate one, the other starts to grow out of hand. some people have had urinary tract infections from fungi because they took too many antibiotics.  not a pretty sight.   just take antibiotics when you have a bacterial infection. if you don't know the origin of your infection (if it is bacterial or viral), your doctor may initially prescribe antibiotics until he can confirm the origin of the infection in lab work.