How can I stop having very bad body odor?

Find cause to RX. If your odor is caused by poor hygiene, you should be able to improve it by washing body, hair and clothes more regularly and using deodorants. Sometimes it can be from dental problems so if it seems related to the mouth, see your dentist. Sometimes it is related to what you have eaten/drunk so be careful to avoid strong-smelling things even alcohol. If these measures fail, see your doctor.