You absolutely can, so long as you follow one simple guideline. Don’t add boiling water into your protein shaker. Even though most plastics are made completely BPA-free, plastic is just a really hardened and condensed oil. Adding boiling water can extract elements out of the plastic and dissolve it into your food, which is really not good for your health. So long as you try to stay under a threshold of 200 F (which is easy), you can add warm liquids to your shaker.\r