Final answer:
Hypotonic fluids should not be given to infants or head injury patients because they may lead to cerebral edema. Maintaining osmotic balance is crucial in these cases and should be managed by healthcare professionals.
Step-by-step explanation:
The substance not to be given to infants or head injury patients, as it may cause cerebral edema, is generally speaking hypotonic fluids, like water or certain types of intravenous fluids. These can lead to water intoxication and swelling of the brain tissue, exacerbating the injury or causing serious complications in infants. In patients with head injuries, maintaining the proper osmotic balance is critical to avoid increasing intracranial pressure, which can be deadly. Specific types of medication or treatment options should always be discussed with a healthcare professional to ensure the safety and well-being of these vulnerable groups.