Even though a head and neck cancer radiation is one of the most effective ways to handle cancer cells, it comes with a lot of side effects. Some head and neck cancer radiation long term side effects may still be easy to manage though. For instance, a radiation therapy that involves the head and neck has affects on the developing adult teeth that lie within the jawbone. This will cause an eruption of teeth in an abnormal alignment (teeth may be crooked). However, other side effects can be an extreme case.