Danger of infant taking 25mg of benadryl (diphenhydramine)?

Sleepiness: If the infant took one dose, they may get very sleepy. Try to hydrate the baby if he/she is awake as much as you can and if you notice any trouble breathing take them to the er, but generally it should be ok and will resolve on its own.

Depends on variables: This would be a proper dose for a 40 lb kid & a double dose at 20lb. Overdose tends to cause nervous system symptoms. Sedation, nausea & vomiting are common. Some infants may have the opposite effect & this provides over-stimulation & excitation in some.