Should additional Diphenhydramine be provided if a patient took a smaller dose than in the directive?

Diphenhydramine dosing for allergic reactions is cumulative rather than episodic. If a patient has already received 25 mg and remains symptomatic, the appropriate approach is to administer an additional 25 mg to reach a total dose of 50 mg. It is not necessary, nor recommended, to administer another full 50 mg dose. If the 25 mg was sufficient to manage symptoms, no additional dosing is required. 

From a clinical perspective, diphenhydramine provides H1 receptor antagonism to reduce histamine-mediated cutaneous and mucosal symptoms. Once adequate receptor blockade is achieved, additional dosing does not meaningfully improve efficacy and instead increases the risk of adverse effects such as sedation, anticholinergic effects, hypotension, and delirium. These risks are particularly relevant in patients who are already physiologically stressed.

It is also important to reinforce that diphenhydramine is adjunctive therapy only. Ongoing or worsening airway compromise, hypotension, or respiratory symptoms should prompt reassessment and repeat epinephrine administration rather than escalation of antihistamine dosing.