Acute anticholinergic medicines and cognitive function

Anticholinergic medicines reduce the action of acetylcholine, a neurotransmitter involved in attention, learning, memory and visual-spatial processing. Their short-term cognitive effects can emerge within hours, particularly when several medicines with anticholinergic properties are taken together.

The response is often dose-related, yet it is not uniform. Older adults, people with dementia, patients who are acutely unwell and those with reduced kidney or liver function may experience confusion, slowed thinking or impaired concentration after a relatively modest exposure.

This issue sits at the meeting point of neuroscience and humane clinical care. A medicine that relieves bladder urgency, nausea, allergy symptoms or muscle spasm may still affect a person’s ability to follow instructions, drive safely or manage medicines independently.

For Australian clinicians, the topic has practical relevance across hospital wards, residential aged care, general practice and community pharmacies. Products available through the PBS, private prescriptions and over-the-counter pharmacy sales can contribute to a patient’s total anticholinergic burden.

Medicine group	Common acute cognitive effects	People needing particular care
Sedating antihistamines	Drowsiness, slowed attention, poor working memory	Older adults, drivers, people using alcohol or sedatives
Bladder antimuscarinics	Inattention, confusion, memory difficulty	Patients with cognitive impairment or constipation
Tricyclic antidepressants	Mental slowing, blurred vision, disorientation	People with falls risk, arrhythmia or polypharmacy
Antiparkinsonian anticholinergics	Confusion, agitation, perceptual changes	Older people and patients with Parkinson’s disease
Antispasmodics and antiemetics	Reduced alertness, impaired new learning	People with dehydration or acute illness

Why acetylcholine matters for cognition

Acetylcholine supports communication between brain networks that regulate alertness, encoding and retrieval of information. Blocking muscarinic receptors can disrupt these processes, producing measurable changes in sustained attention, reaction time and short-term memory even when a person does not appear overtly sedated.

The effect can resemble ordinary tiredness at first. A patient may lose track of a conversation, repeat a question, misread medication directions or struggle with a familiar task. In hospital, these subtle changes can be mistaken for poor cooperation, pain, sleep deprivation or the early stages of delirium.

Medicines that can produce a rapid effect

Diphenhydramine and doxylamine are familiar examples of sedating antihistamines, while oxybutynin, tricyclic antidepressants and some antiemetics can add substantial antimuscarinic activity. In Australia, a person may purchase a sleep or cold-and-flu product from a pharmacy without realising that it has cognitive consequences when combined with prescribed treatment.

The risk rises when medicines with similar properties are layered together. A patient taking an antidepressant, bladder medicine and travel sickness product may have a much higher overall anticholinergic load than the medication list suggests. A careful medicines reconciliation should include non-prescription products, recent dose changes and occasional use.

Delirium, ageing and individual vulnerability

Acute cognitive toxicity may present as delirium: a rapid change in attention, awareness and thinking that fluctuates during the day. Hallucinations, agitation, urinary retention, dry mouth, blurred vision and reduced sweating can strengthen suspicion, although a quiet, sleepy presentation is also possible.

Older people are especially vulnerable because ageing may reduce drug clearance and the brain’s ability to compensate for cholinergic disruption. Dehydration during a Brisbane heatwave, infection in a residential aged-care facility or postoperative stress in a Sydney hospital can further lower the threshold for confusion.

Assessing cognition in clinical practice

A brief bedside assessment should examine attention, orientation, new learning and the patient’s ability to maintain a conversation. Tools such as the 4AT or the Confusion Assessment Method can support delirium screening, but a baseline from family, carers or previous clinical notes is often essential.

Neuropsychological testing may show slowed processing, reduced immediate recall and poorer divided attention. Results need to be interpreted alongside sleep, mood, pain, hearing, language and cultural factors. In a multilingual Melbourne community, testing in English may underestimate a patient’s usual abilities if language support is not considered.

Safer prescribing and deprescribing

Management begins with identifying the likely offending medicine and weighing its benefit against the severity of cognitive symptoms. Clinicians may reduce the dose, stop a non-essential product or choose an alternative with less anticholinergic activity, while monitoring for withdrawal or recurrence of the original condition.

A pharmacist-led review can be particularly valuable in Australian general practice and aged care, where multiple prescribers may be involved. The patient’s partner or carer should receive clear instructions, especially if the person drives, lives alone or manages medicines in a weekly dose administration aid.

Translating evidence into humane care

Good care avoids treating confusion as a behavioural problem. A calm environment, visible clocks, familiar staff, adequate hydration, hearing aids and regular orientation can reduce distress while the medication review takes place. Family observations may reveal that the cognitive change began soon after a new tablet or dose increase.

Conference discussions linking neuroscience with clinical practice also highlight the importance of accessible environments. Planning for patients with cognitive or mobility needs is supported by guidance on venue accessibility, a principle that applies equally to Australian clinics and hospitals.

The goal is proportionate prescribing: preserve symptom control while protecting attention, independence and dignity. For patients in Perth, Adelaide or remote communities, this may require practical coordination between local doctors, pharmacists, Aboriginal health services, carers and hospital teams so that cognitive changes are recognised early and acted on safely.