What if I ate something poisonous and I don't have any medicine?

Don't eat something poisonous is the first rule. If you do, then induce vomiting. Test a very small amount instead of eating a whole lot of something. Rub a small amount on your lips. Wait a few hours, if no discomfort, burning sensation on lips or other ill effects move on to tongue. A small amount rubbed on tongue. Wait a few hours. If no ill effects or burning sensation etc move on to swallowing a small amount. If no ill effects, then swallow a bigger amount. If you're stranded on a deserted island, you need to keep your wits about you in what you choose to eat.