Most dogs love peanut butter, but the next time you give your pooch a peanut buttery treat, be sure it doesn't have xylitol in it. This sugar substitute can be harmful to them, since it can cause a steep spike in their blood sugar if they ingest it. It's important to read the labels carefully. Many peanut butters that aren't even marked low-sugar can have xylitol. The best thing to do is make your own natural peanut butter or choose butters that are made from 100 percent peanuts.