The usual adult dose of doxylamine for allergic rhinitis or allergic conjunctivitis is 10 milligrams every four-to-six hours to a maximum of six doses per day, according to Drugs.com. For sleeplessness or insomnia, the dose for people 16 and over is 25 milligrams once before bedtime.
In children over the age of 12, the dose of doxylamine for allergic symptoms is the same as that for adults, states Drugs.com. In children between the ages of 6 and 12 years, it is 5 milligrams every four-to-six hours, but not more than six doses per day. Doctors do not recommend doxylamine for children under 6, nor do they recommend the 25-milligram dose for children under age 12.