Taking Allegra and Benadryl in close succession is a common question for people seeking relief from persistent allergy symptoms. Both medications work by blocking histamine, the chemical released by the immune system that triggers allergic reactions like sneezing, itching, and watery eyes. Allegra (fexofenadine) is a modern, non-drowsy option, while Benadryl (diphenhydramine) is an older medication known for its sedating effects. Understanding the differences in how these drugs function is necessary to appreciate the risks involved in combining them.
Understanding Antihistamine Generations
The difference between Allegra and Benadryl is rooted in their classification as first- or second-generation antihistamines. Antihistamines function as H1 receptor blockers, preventing histamine from binding to receptors on cells throughout the body. Benadryl (diphenhydramine) is a first-generation antihistamine, developed decades ago, that is highly lipophilic.
This lipophilic nature allows diphenhydramine to readily cross the blood-brain barrier. Once inside the central nervous system (CNS), the drug blocks H1 receptors there, leading directly to side effects such as drowsiness and impaired coordination. Allegra (fexofenadine), in contrast, is a second-generation antihistamine designed to be less lipophilic.
Fexofenadine does not easily cross the blood-brain barrier, which allows it to target peripheral H1 receptors responsible for allergy symptoms with minimal entry into the CNS. This mechanism explains why Allegra is marketed as a non-drowsy allergy relief option. The newer generation drugs are more selective and have a different side effect profile.
The Primary Risk of Combining Medications
The risk when taking Benadryl after Allegra stems from the additive effect of combining two drugs that act on the same biological system. While Allegra is minimally sedating, taking Benadryl introduces a significant risk of central nervous system depression. Both medications contribute to the overall antihistamine load in the body, which increases the sedative properties of diphenhydramine.
The combination can lead to excessive drowsiness, profound sedation, and dizziness, impairing a person’s ability to drive or perform tasks requiring mental alertness. Diphenhydramine also has anticholinergic effects, which can cause side effects like dry mouth, blurred vision, and difficulty urinating, and combining it with another antihistamine can heighten these risks. The simultaneous presence of two H1 blockers is generally discouraged.
Healthcare providers typically recommend against combining two oral antihistamines unless it is under specific medical direction for severe symptoms.
Safe Dosing and Timing Guidelines
To manage persistent allergy symptoms safely, choose a single, long-acting second-generation antihistamine like Allegra and use it consistently as directed. If additional relief is required after taking Allegra, avoid taking Benadryl immediately afterward due to the increased risk of over-sedation. A safer approach involves staggering the doses to allow the first drug to clear the body significantly before introducing a second one.
Since Benadryl’s effects typically last between four to six hours, waiting for at least that long before taking another dose of any antihistamine helps mitigate the risk of excessive sedation. If severe nighttime symptoms require a sleep aid, the sedating effect of Benadryl is sometimes utilized, but this should be taken only at bedtime.
Consulting with a pharmacist or physician is important before creating a regimen that involves overlapping allergy medications. They can assess the specific symptoms and recommend a safer alternative, such as a nasal steroid spray, which works through a different mechanism and does not contribute to the sedative effect.