The most commonly recommended antihistamine as premedication for patients with a history of allergies prior to dental procedures is Diphenhydramine.

- Diphenhydramine is a first-generation antihistamine known for its strong H1 receptor antagonism and effective prevention of allergic reactions.
- It has sedative properties, which can be beneficial in reducing anxiety during dental procedures.
- Due to its rapid onset of action and ability to be administered orally or intramuscularly, it is preferred for immediate premedication.
- Unlike second-generation antihistamines such as Cetirizine, Loratadine, and Fexofenadine, diphenhydramine can cross the blood-brain barrier, leading to sedation, which is useful in procedural settings.
- Second-generation antihistamines are typically less sedating and have a slower onset, making them less effective for acute premedication use.

Thus, Diphenhydramine is preferred for its quick onset, sedative effect, and strong antihistamine action, ensuring better protection against allergic reactions during dental interventions.

Reference: Gray’s Anatomy of the Human Body, Volume 1, Chapter 15 – Allergic Reactions and Antihistamines, p. 245-247