Anticholinergic Equivalence in Psychotropic Medications: A Guide for Psychiatrists

Anticholinergic side effects from psychotropic medications are common and can lead to significant adverse events, including cognitive impairment and falls, particularly in vulnerable populations like the elderly. The cumulative anticholinergic burden from multiple medications is a critical concern associated with poorer clinical outcomes. Quantifying this burden is essential for safer prescribing.

This article developed an anticholinergic equivalence (AE) table for various psychotropic medications. Diphenhydramine (AE=1) was used as the reference standard. AE values for other drugs were derived from their M1 muscarinic receptor binding affinities (Ki) relative to diphenhydramine. This allows estimation of the diphenhydramine equivalent burden per milligram of a given medication. Antihistaminic properties were also reviewed.

An AE table was generated, detailing the anticholinergic potency of numerous psychotropic agents. Values varied significantly, with older tricyclic antidepressants (eg, amitriptyline AE: 8.99) and some antipsychotics (eg, clozapine AE: 6.67, olanzapine AE: 3.08) showing high anticholinergic equivalence. Many sedating medications (eg, quetiapine, mirtazapine) are noted to have potent antihistaminic but low anticholinergic properties, clarifying that sedation is not always due to anticholinergic effects.

The anticholinergic equivalence table provides a practical, pharmacologically based tool for psychiatrists to quantify and compare the anticholinergic potential of psychotropic medications. This can aid in minimizing cumulative anticholinergic burden, making more informed prescribing decisions, and ultimately enhancing patient safety and therapeutic outcomes, especially in high-risk groups.