Potential side effects of taking diphenhydramine hydrochloride 25 mg include drowsiness, irritability, paresthesia, disturbed coordination and agitation, according to the U.S. National Highway Traffic Safety Administration. Blurred vision, constipation, dry mouth and upset stomach are additional side effects, advises WebMD.
When combined with hypnotics, tranquilizers, diazepam, alcohol or other depressants, the effects of diphenhydramine are magnified, notes the U.S. National Highway Traffic Safety Administration. Monoamine oxidase inhibitors intensify and extend the drug's anticholinergic effects, such as dry mouth.
As an antagonist, diphenhydramine can both stimulate users and cause depression, indicates the U.S. National Highway Traffic Safety Administration. Stimulation effects include nervousness, sleeplessness and restlessness. Depression effects include somnolence, slower reaction time and decreased alertness.
Diphenhydramine is used as a sleep aid, cough suppressant and antihistamine to treat allergy symptoms, explains the U.S. National Highway Traffic Safety Administration. The drug is administered orally, topically or through an injection.
Diphenhydramine is not for use in children under 12 years of age, advises the U.S. National Library of Medicine. Additionally, it is not advisable to use diphenhydramine with other products that contain diphenhydramine, or with other drugs that cause drowsiness. Individuals with heart disease, glaucoma, an enlarged prostate gland or breathing problems, such as emphysema, should consult a doctor before taking diphenhydramine.