The Bac water allows you to keep, in this case, HGH, longer. When you use sterile water the instructions will tell you that you have to use the whole product in two days. This is because water is a breeding ground for bacteria. In a multiple-use vial every time you penetrate that septum you risk contamination. Some claim that there always is a bit of contamination every time you penetrate that septum. It's so slight that it is going to spoil the product that the alcohol in the Bac water prevents it from spreading and growing. That is not the case with sterile water. I think two days is very conservative but companies will always err on the side of safety. Bac water is so cheap here that I would always use Bac water.