Concussions from head injuries are unfortunately all too common. Having too many (and no one really knows what that magic number is) can lead to permanent brain damage with seizure and personality changes.
After a head injury which knocks you out or causes a severe headache, nausea vomiting or confusion, you should of course go to the ER. Once seen and assuming you are sent home, you should avoid contact sports for at least one week. In fact, children should avoid contact sports for more time say at least two weeks. While there are still no firm guidelines about how long to avoid contact sports, longer is better.
Not only should avoid sports, but you should also let your brain rest. By this I mean avoid stress, computers, video games, reading and writing for several days. This allows the brain to take it easy and recover from a concussion.
Expect to experience occasional confusion, headaches and dizziness sometimes up to one year later. Some even permanently lose their sense of smell.
Avoiding concussions is not so easy. Always remember to wear sports specific head protection when necessary. If you aren’t feeling well, then seriously consider holding off with contact sports or sports with potential for head injury (mountain biking for example).