The ingredients in the Pre-Workout are extremely hygroscopic. The moment you break the seal, the powder will start pulling in moisture from the air (even with the lid on.) Some caking or clumping is normal after a few weeks, and is easily remedied by shaking the bottle vigorously before use.
We recommend finishing the bottle within 60 days after opening. If you use the Pre-Workout infrequently, or if you're juggling multiple flavors at once, you can store the bottles in the freezer to help prevent moisture build up.