It's very plausible that anxiety causes heart disease, anxiety causes insomnia, and insomnia leads people to use melatonin. Same with diphenhydramine, overactive inflammatory response causes allergies, allergic people take allergy meds, and too much inflammation contributes to dementia.
Association studies too easily get interpreted as X causes Y. Maybe that's true, but not necessarily.
Association studies too easily get interpreted as X causes Y. Maybe that's true, but not necessarily.