Absolutely! All Hammer Nutrition fuels are completely compatible with one another, so you can use them interchangeably as desired. This is especially beneficial in ultra-endurance events as it provides a greater variety of quality fuels to choose from. For example, you can use Perpetuem and/or Sustained Energy from start to finish, or you can occasionally switch to HEED and/or Hammer Gel to add variety. However, you should meet at least two-thirds of your fueling requirements from Perpetuem or Sustained Energy. When you use Hammer Gel and HEED for events longer than two hours, you do not need to start with them and then switch to Sustained Energy or Perpetuem. You can use Hammer Gel and/or HEED at any time during your workout. An extra flask of Hammer Gel in your pocket can save the day if you have already drained your Sustained Energy or Perpetuem mix and you begin to flag with several miles still to go. It will give you a quick pick-up just when you need it, even if it's many hours into your event.