Don't shake vigorously. Aggressive shaking can cause peptides to denature (unfold and lose activity) by creating excessive foam and air-liquid interfaces. Instead, direct the BAC water stream down the side of the vial, then gently swirl or roll the vial between your palms. Most peptides dissolve within a minute or two. If powder remains after gentle swirling, let it sit in the refrigerator for 15-30 minutes and try again—forcing it with shaking isn't worth the risk.