Suppose a medicine killed all the bacteria in your body. how might this affect vitamin production in your body?

If the medicine killed all the bacteria, you wouldn't be able to sustain other bad bacteria, for your body holds the amount of bacteria that is needed to live. Your vitamin production decrease, and you probably wouldn't be able to survive for long. One of your needs to survive for your body is to be able to have bacteria to fight off viruses, to protect your body from the outside, some bacteria is needed to give nutrients or signals to the brain.