They can’t eat all types of peanut butter. If a brand of peanut butter that contains xylitol is fed to a dog, it will make them sick and potentially kill them. Xylitol is used as an artificial sweetener, and is very toxic to dogs. Don’t ever feed your pets human food without checking what’s in the ingredients label.
There are other sugar alcohols out there that are just as dangerous, and are slipped into all kinds of foods, especially those designed to be lower in sugar (whether it’s branded that way or not), so ALWAYS check the label before feeding pets any processed human foods, as u/Catpurple mentioned above.
Man, that shouldn’t be acceptable for humans, either. I’m all for having low-calorie options, but I think that humanity can use an approach that doesn’t consist of making random foods surprise laxatives.