Which characteristic is commonly associated with narcotics?

Narcotics, also known as opioids, are a class of drugs commonly used for pain relief. One of the hallmark characteristics of narcotics is respiratory depression. This occurs because narcotics act on the brain's respiratory centers, reducing the respiratory rate and depth of breathing. This effect can be particularly dangerous, especially in cases of overdose, as it may lead to insufficient oxygenation and respiratory failure.

In contrast, increased alertness is typically associated with stimulants, rather than narcotics. Severe headaches can occur due to various reasons but are not a defining characteristic of narcotics. Elevated heart rate is also not a typical effect of narcotics; in fact, they may cause a decrease in heart rate due to their sedative effects. Therefore, respiratory depression is a crucial and defining feature of narcotic use and its impact on the central nervous system.