Doxylamine succinate is recognized as safe to administer to healthy adults, though it does come with the usual dosage limitations and toxicity warnings of other antihistamines. What’s important to remember is that the standard dosage of doxylamine when used as a sleep aid is around 25mg, which is substantially less than the median lethal dose, estimated to be ~500 mg/kg in humans which relates to a dose of approximately 35 grams.
Overdoses from doxylamine have been reported in the past. As with all night aids and herbal sleep remedies, responsible use is essential to achieving successful results. Please remember to follow all directions provided on the labeling, and keep your nighttime allergy medicine away from children and pets.