Excess, unecessary or incomplete dosage of antibiotics may lead to:
Resistance of bacteria against that antibiotic
As bacteria are living beings, they also evolve with respect to their outside environment. Antibiotics are chemicals that kill bacteria or slow down their growth. If bacteria is exposed to an antibiotic then the surviving population begins to develop immunity against the antibiotic. This immunity is passed on to successive generations and after few generations, the antiobiotic eventually becomes inefffective.