A student misuses an opioid painkiller by taking a higher dose than prescribed. What is a serious and immediate effect this can have on the body?

Opioid painkillers (such as morphine, codeine, tramadol, and fentanyl) act on opioid receptors in the brain and brainstem. At prescribed doses they relieve pain, but at higher-than-prescribed doses they overstimulate these receptors, causing respiratory depression - a dangerous slowing or cessation of breathing.

The brainstem contains the respiratory centre that automatically controls the rate and depth of breathing. Opioids suppress this centre in a dose-dependent manner: the higher the dose, the greater the suppression. In severe cases, breathing can slow to the point where the body no longer receives enough oxygen, leading to hypoxia, loss of consciousness, and potentially death. This is the primary mechanism by which opioid overdoses are fatal.

The other options describe effects that are the opposite of what opioids actually do:

• A sudden increase in muscle strength does not occur; opioids cause muscle relaxation.
• Improved coordination and balance is incorrect; opioids impair motor coordination, cause dizziness, and reduce alertness.
• An increase in heart rate and energy is wrong; opioids typically slow the heart rate and produce sedation, not stimulation.