The immediate effects of a head injury can include dementia symptoms, such as confusion, memory loss, and changes in speech, vision and personality. Depending on the severity of your injury, these symptoms may clear up quickly, last a long time or never go away completely. However, such symptoms that begin soon after your injury generally don't get worse over time as happens with Alzheimer's disease.
Certain types of head injuries, however, may increase your risk of developing Alzheimer's disease or other dementias later in life. The greatest increase in future dementia risk seems to occur after a severe head injury that knocks you out for more than 24 hours. A moderately serious head injury that causes unconsciousness for more than 30 minutes, but less than 24 hours, also seems to increase risk to a smaller extent.
There's no evidence that a single mild head injury that doesn't knock you out, or that knocks you out for less than 30 minutes, increases your risk of dementia. However, repeated mild injuries may increase risk of future problems with thinking and reasoning.
You're likely at greatest risk of developing dementia or Alzheimer's later in life, post-head injury, if you also have other risk factors. For example, carrying one form of the apolipoprotein E (APOE) gene increases the risk of Alzheimer's in any individual. A head injury in such a person would increase his or her risk further.
It's important to note that many people who sustain a severe head injury never develop Alzheimer's disease or later dementia. More research is needed to understand the link.