Anticholinergics: Safety concerns

Last updated: May 13, 2022

Anticholinergic medicines block the action of acetylcholine.

Acetylcholine is a neurotransmitter – a chemical that enables nerves to communicate with organs and with other nerves. It's particularly important in the parasympathetic nervous system, the arm of the autonomic nervous system concerned with body maintenance and baseline functioning. So acetylcholine mediates activities such as bladder emptying, slowing of the heart, saliva production, control of body temperature, focusing of the eye, and peristalsis. Acetylcholine is also found in the brain where, amongst other actions, it helps alertness, concentration, and learning.

Medicines that oppose the action of acetylcholine are traditionally called anticholinergics. These medicines are also called muscarinic antagonists or antimuscarinics because the site where acetylcholine mediates many of its actions is known as the muscarinic receptor.

Given the widespread role of acetylcholine in controlling basic body functions, anticholinergic medicines can cause a broad range of side effects many of which are summarised in the infographic below:

You might have been taught an old rhyme at medical school to remember some of these:

Blind as a bat, hot as a hare, red as a beet, dry as a bone, mad as a hatter. 

There's another cruder, but maybe more memorable one:

Can't see, can't spit,
Can't wee, can't  💩

But don't forget the other anticholinergic side effects such as sedation and tachycardia which aren't mentioned in either of these mnemonics.

As we saw in the case study at the beginning of this tutorial, some of these side effects may present in an oblique way. Drug-induced dry mouth may be a factor in causing dental decay, a patient's blurred vision might be interpreted as needing a new prescription for glasses, and feeling hot as perhaps a menopausal symptom. Once you start identifying patients taking anticholinergic medicines, it may prompt you to take a more holistic view of the aetiology of apparently unrelated clusters of symptoms.

There are some significant longer term safety concerns from anticholinergics as well, such as falls and dementia, which we discuss later.

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