Atropine

In controlled medical use atropine is valuable and often lifesaving, but it has a narrow margin; too much causes the full anticholinergic toxidrome (blurred vision, delirium, flushing, dangerous overheating, dry mouth), along with racing heart, urinary retention, seizures, and death. As a deliriant it produces realistic hallucinations mistaken for reality, confusion, and memory loss, an experience that is dangerous and generally deeply unpleasant. Do not combine it with other anticholinergics or antihistamines, which stacks the burden, and be aware elderly people and long-term anticholinergic use carry dementia-risk associations. Not medical advice.

Atropine is a competitive muscarinic antagonist, so its interactions are mostly a matter of adding or subtracting antimuscarinic tone. Drugs with their own anticholinergic activity stack with it: first generation antihistamines, tricyclic antidepressants, low potency antipsychotics such as chlorpromazine, antiparkinsonian agents like benztropine, and gut antispasmodics. The combined burden produces the classic toxidrome of dry mouth, blurred vision, urinary retention, tachycardia, impaired sweating with hyperthermia, and in older adults frank delirium.

The mirror image is antagonism. Atropine directly opposes cholinesterase inhibitors, which is the point in organophosphate poisoning but counterproductive alongside pyridostigmine in myasthenia gravis or donepezil in dementia, where each blunts the other.

Slowed gastric emptying changes how other oral drugs are absorbed, delaying onset for some and increasing mucosal contact time for others; solid potassium chloride formulations are the usual example, where prolonged contact raises the risk of gut ulceration. Reduced saliva also impairs dissolution of sublingual tablets such as nitroglycerin.

Checking a whole stack? Run it through interactions + stacks.