What are the uses and dosages of anticholinergic (Anti-Cholinergic) medications?

Anticholinergic Medications: Uses and Dosages

Anticholinergic medications block muscarinic acetylcholine receptors and are used across multiple clinical conditions, though their use requires careful consideration of their significant adverse effect profile, particularly in older adults.

Primary Clinical Uses

Gastrointestinal Disorders

Antiemetic therapy is a major indication for anticholinergics with antihistaminergic properties:

• Promethazine: 12.5–25 mg orally or rectally every 4–6 hours during episodes for nausea/vomiting, though peripheral IV administration can cause tissue injury including gangrene or thrombophlebitis 1
• Prochlorperazine: 5–10 mg every 6–8 hours or 25 mg suppository every 12 hours, with caution in patients with history of leukopenia, neutropenia, dementia, glaucoma, or seizure disorder 1
• Diphenhydramine: 12.5–25 mg every 4–6 hours during episodes, though caution is warranted in older adults with glaucoma, benign prostatic hypertrophy, ischemic heart disease, or hypertension 1
• Scopolamine: 1.5 mg patch every 3 days for gastroparesis-related nausea, despite lack of supporting clinical studies 1

Irritable bowel syndrome and functional bowel disorders:

• Dicyclomine: Initial dose of 40 mg four times daily (160 mg total daily), though 61% of patients experience anticholinergic side effects and 9% discontinue due to adverse effects 2

Movement Disorders

Parkinson's disease represents a traditional indication, though anticholinergics have been largely supplanted by dopaminergic agents:

• Trihexyphenidyl: Start with 1 mg on day one, increase by 2 mg increments every 3–5 days to a total of 6–10 mg daily (some postencephalitic patients may require 12–15 mg daily), divided into 3–4 doses 3, 4
• Benztropine, biperiden, orphenadrine, procyclidine: All demonstrate efficacy over placebo for motor symptoms, though specific dosing varies 5

Drug-induced extrapyramidal symptoms from antipsychotics:

• Trihexyphenidyl: 5–15 mg daily total dose, though some patients controlled with as little as 1 mg daily; commence with 1 mg and progressively increase if needed 3

Urological Conditions

Overactive bladder and enuresis:

• Oxybutynin: 5 mg at bedtime, may double if needed 1
• Tolterodine: 2 mg at bedtime, may double if needed 1
• Propiverine: 0.4 mg/kg at bedtime 1

These are only indicated after failure of standard behavioral treatments, exclusion of constipation, and confirmation of no post-void residual urine 1.

Psychiatric Applications

Cognitive symptom management in schizophrenia requires minimizing anticholinergic burden, as clozapine, olanzapine, and quetiapine have the highest central anticholinergic activity 1.

Common Adverse Effects

The classic anticholinergic syndrome is described as "Red as a beet, dry as a bone, hot as a hare, blind as a bat, mad as a hatter, full as a flask" 1.

Peripheral Effects

• Dry mouth, dry eyes, blurred vision (mydriasis), constipation, urinary retention 2, 6
• Tachycardia, hypertension 1
• Decreased sweating, hyperthermia 1, 2

Central Nervous System Effects

• Cognitive impairment: Confusion, delirium, agitation, hallucinations, memory impairment 2, 6, 7
• Sedation, somnolence, oversedation 1, 2
• In severe cases: amnesia, disorientation, pseudodementia 2

Serious Complications

• Neuropsychiatric events are more common than lack of efficacy as reasons for withdrawal 5
• Increased risk of falls, functional decline, institutionalization, and mortality in older adults 6, 7, 8

Critical Clinical Considerations

High-Risk Populations

Older adults are particularly vulnerable to anticholinergic adverse effects:

• One-third to one-half of medicines prescribed to older people have anticholinergic activity 7
• Anticholinergic burden predicts cognitive and functional impairments 7, 8
• Exercise extreme caution in patients with dementia, glaucoma, benign prostatic hypertrophy, or cardiovascular disease 1

Drug Interactions and Cumulative Burden

Cumulative anticholinergic burden from multiple medications significantly increases adverse outcomes:

• Many commonly prescribed drugs have anticholinergic properties: antispasmodics, bronchodilators, antiarrhythmics, antihistamines, antihypertensives, skeletal muscle relaxants, and psychotropics 6
• The anticholinergic burden should be minimized by avoiding, reducing doses, or deprescribing where clinically possible 7, 8

Monitoring Requirements

For dicyclomine: Monitor for dose-related reversible effects; 46% of patients with side effects required dose reduction from 160 mg to average 90 mg daily 2

For urological anticholinergics: Monitor for constipation (most bothersome side effect) and post-void residual urine (risk of UTIs); maintain sound voiding habits 1

For Parkinson's disease: Abrupt withdrawal may cause acute exacerbation of symptoms or neuroleptic malignant syndrome 3

Practical Prescribing Algorithm

1. Confirm indication is appropriate and non-pharmacological options have been exhausted
2. Screen for contraindications: glaucoma, urinary retention, severe constipation, cognitive impairment, cardiac arrhythmias 2, 6
3. Start low, go slow: Use lowest effective dose, particularly in older adults 3, 7
4. Monitor closely: Assess for cognitive changes, urinary retention, constipation, and cardiovascular effects 1, 2, 6
5. Minimize cumulative burden: Review all medications for anticholinergic properties 7, 8
6. Regular reassessment: Attempt dose reduction or discontinuation when clinically appropriate 1, 7