Raw kidney beans contain phytohaemagglutinin, a toxic compound that causes severe nausea, vomiting, and abdominal pain. You’ll be fine if you cook them properly at a rolling boil for at least 10 minutes, but slow cookers on low heat won’t get hot enough to destroy the toxin. Canned beans are already processed safely, so those are your safest bet if you’re not confident in your cooking method.