Before modern medicine, one of the number one causes of death was infection. We now have antibiotics. When stocking up on antibiotics, it is important to choose the right one. Many studies have shown Amoxicillin to be more effective than Penicillin. Amoxicillin also does not need to be taken as long as Penicillin making it a much better candidate for stockpiling since a stockpile will last longer.
The problem is that if you go to your local healthcare practitioner and tell him to give you a lifetime supply of Amoxicillin because you are prepping for the apocalypse, he/she will likely not only turn you down but may declare you mentally unstable. Even if you find a doctor that understands prepping, you will still have to deal with the outlandish cost of buying pharmaceuticals in bulk. When you need a solution for this, look to the fish. Fish antibiotics are available to anyone online for cheap. The neat thing is that they are identical to the human medication counterpart, and available in human doses.