How long does it take a 25mg benadryl (diphenhydramine) pill to start working on allergy?

25-45 min: It usually takes from 25 to 45 for a pill to start to make effect has to be dissolved and absorb into the blood stream to make effect.

Should: Start working within the hour. But it is a low dose and sedating. You might try an Allegra or Claritin (loratadine) ( store brand ok) that is once a day and once in your system you won't be on a roller coaster of it working or not as long as you take it regularly.