Head injuries can cause complications quite a time after they occur
Nearly 80% of hospitals are not giving the right amount of information to patients discharged after a head injury, according to research.
Brain injury association Headway said 14 of 18 A&E departments in Wales were providing less than half the amount of written information recommended.
The group said it posed a risk of further complications to those with minor head injuries.
The assembly government could not comment until receiving the report.
Information levels are set out in health body NICE guidelines.
Headway added almost half of A&E departments were not providing any information whatsoever on post-concussion syndrome to patients, and two-thirds failed to tell patients not to drive immediately after injury.
The data was gathered by researchers at Warwick Medical School.
Dave Maggs, development manager for Headway in Wales, said: "When a minor head injury has occurred, it may take some time for the symptoms to show.
"Even if the knock appeared innocuous at the time, it is essential to take each case seriously.
"The fact is, even a minor bang to the head can have major consequences and it is vital that patients leave hospital with high-quality, detailed information on the symptoms to look out for.
"Without being warned, a patient wouldn't know they shouldn't be left alone for 48 hours after leaving hospital, or the potentially serious implications of suffering weakness in their arms or legs.
"So it is essential that we continue to work with the hospitals in Wales to ensure the departments know exactly what information needs to be provided to patients, and when, to improve the situation."
Dr Carol Hawley, who collected the data, said even if patients were given the information verbally, it may be hard for them to take it in at that time.
"It is important that adequate and consistent written information is also supplied by all hospitals across the UK," she added.
A Welsh Assembly Government spokesman said: "We haven't received the report and we can't comment until we do so."