Benadryl (diphenhydramine)? It probably shouldn't be used for prolonged periods of time (associated with cognitive decline and dementia among the elderly).
Yeah the anticholinergic action has some association with dementia. Lots of medications have some kind of "Anticholinergic burden", drugs that cause low-level cognitive impairment, there's a list.
But I'm almost sure Benadryl isn't good for stress management on the simple basis that functioning cholinergic signaling is the inhibitory agent of the sympathetic nervous system. On any degree of anticholinergic effect you're more of in a flight-or-flight humor. This is apparent on acute effect (overdose,) the anticholinergic toxidrome symptomatology mnemonic goes like this: red as a beet, blind as a bat, mad as hatter, hot as a hare, dry as a bone - all something you could get with amphetamine (a dominant sympathetic nervous system). Do you reckon amphetamine relaxing?
Associated with cognitive decline could just indicate that the people using it are more sickly or have more trouble sleeping, both of which are more strongly correlated with cognitive decline.