4.6. Other adverse effects

Opioids can cause bladder dysfunction and the effects can range from difficulty in voiding to urinary retention; these adverse effects often occur after surgery. Tolerance1 to these opioid effects develops on longer use.

Constriction of the pupil—miosis—results from opioid stimulation of the parasympathetic nervous system; pinpoint pupils are characteristic of opioid poisoning.

Factors which increase risk

Urinary retention is more often associated with epidural injection rather than intravenous or intramuscular injection. Take care in those susceptible to urinary disorders including patients with prostatic hypertrophy and the elderly. Concurrent use of an antimuscarinic (anticholinergic) drug may increase the risk of urinary retention.


In severe cases of urinary retention, bladder catheterisation may be needed. The opioid antagonist2 naloxone may be able to reverse the urinary effects of opioids.

  1. Tolerance follows repetitive exposure to a drug, leading to a reduction of the pharmacological effect of that drug
  2. A substance that binds to a receptor but produces no effect and inhibits an agonist from binding to the receptor