4.2. Central nervous system and psychiatric effects

man with his hand raised to his head Drowsiness and sedation are very common side-effects of opioids. Patients may feel confused and their ability to think clearly, impaired; high doses of opioids increase the risk of delirium. Dizziness is also a frequent side effect. Mood alteration with opioids can range from euphoria and elation to dysphoria (unease and unhappiness). CNS adverse effects generally settle as tolerance1 develops.

Hyperalgesia—increased sensitivity to pain—can follow long-term use of high doses of an opioid. Rarely, convulsions occur with excessive doses (see Overdose), but with some opioids they may occur with therapeutic doses.

Factors which increase risk

Drowsiness and confusion may increase with increasing dose of opioid. These effects also increase if an opioid is taken with medicines with similar side effects, such as antimuscarinic (anticholinergic)2 medicines. Patients with dementia are at greater risk of worsening cognitive impairment. Opioids should be used with care in patients susceptible to convulsions or those with head injuries.

Risk-reduction measures

While the dose of opioid should be adequate, an excessive dose or very rapid dose escalation should be avoided to reduce the risk of CNS effects such as drowsiness and confusion.


CNS side effects of opioids generally diminish without specific treatment. Additional drugs to counteract CNS side effects can be considered, but they may complicate therapy and there are no licensed medicines for this purpose.

Activity 3

What would be your advice on driving for a patient prescribed 5-days’ supply of an opioid for postoperative pain? How might this advice differ for a patient treated for 6 months with a steady dose of transdermal3 opioid patches?

  1. Tolerance follows repetitive exposure to a drug, leading to a reduction of the pharmacological effect of that drug
  2. Reduction or blocking of the effects of parasympathetic nerves; antimuscarinic effects include dry mouth, difficulty swallowing, blurred vision, confusion, palpitations, constipation, and urine retention
  3. An adhesive pad, containing a medicine, which, when affixed to the skin, releases the medicine at a controlled rate into the skin and thence into the bloodstream