Raw beans contain proteins called lectins that break down with cooking. Not all lectins are toxic; some are even beneficial. The lectin in kidney beans, though – called phytohemagglutinin – is harmful at high doses [source: Andrews]. There are other beans that contain phytohemagglutinin, but red kidney beans contain them in the highest concentration by far [source: Medic8]. When it comes to this toxin, it's all about dosage.
Just a handful of raw kidney beans is enough to cause gastrointestinal problems like nausea and vomiting. The more you eat, the more intense your symptoms will be. Some folks have even been hospitalized with red kidney bean poisoning. [source: Medic8]
You don't have to give up your red beans and rice just yet. To destroy the lectins in kidney beans, you just need to soak, drain, and then cook them on the stovetop. Stovetop cooking reaches higher temperatures than a slow cooker, and kidney beans really do need to be boiled to be completely safe. To prepare dry kidney beans safely, soak for at least five hours, drain and rinse, then boil for at least 30 minutes on the stove [source: FDA].