Thanks to its high protein content, soy milk is a typical smoothie alternative for those who can't do dairy. Obviously, you probably already know if you break out in hives when you eat or drink soy products, but some people who aren't allergic still have trouble digesting the protein in soy. If this is you, soy products may cause some, ahem, not-so-fun digestive symptoms.
No need to fret, though. Soy milk can be replaced measure-for-measure with almond milk, coconut milk, or even rice milk. These milks are not as high in protein as dairy or soy milk, however, so make sure you are getting your protein in elsewhere.