Hyoscine butylbromide

A quaternary derivative of hyoscine that blocks muscarinic receptors on the smooth muscle of the gut, biliary and urinary tracts, and also blocks ganglia in the gut wall. Its permanent charge keeps it out of the brain, so it causes little drowsiness, and only a small fraction of an oral dose is absorbed.

Like other antimuscarinics: They occupy muscarinic acetylcholine receptors without activating them, competitively blocking the acetylcholine released by parasympathetic nerves. The heart speeds up, glands dry up, the smooth muscle of the gut, bladder and airways relaxes, and the pupil dilates and stops focusing. Tertiary amines such as atropine enter the brain; quaternary ones such as hyoscine butylbromide do not.

• Heart failure, coronary disease or arrhythmia: the injection can cause serious tachycardia, hypotension or anaphylaxis.
• Contraindicated in myasthenia gravis, narrow-angle glaucoma, megacolon and mechanical or paralytic bowel obstruction.
• The injection is contraindicated in tachycardia and in prostate enlargement with urinary retention.
• Severe or unexplained abdominal pain needs its cause found, not only relief.

Good to know: Not the same as hyoscine hydrobromide, which is used for travel sickness and commonly causes drowsiness; the butylbromide stays out of the brain.

Easily mixed up with direct-acting muscarinic agonists, anticholinesterases and beta-3 agonists.