Benadryl (diphenhydramine) is a common over-the-counter medication used as a first-generation antihistamine. Its primary purpose is to relieve allergy symptoms such as sneezing, runny nose, and itching. Diphenhydramine works by blocking the action of histamine, a substance the body releases during an allergic reaction. The medication is also frequently used for its pronounced sedative properties, which is why it is often included in nighttime cold and sleep aid formulas.
The Standard Dose Compared to Doubling Up
The active ingredient in Benadryl is diphenhydramine, and the typical single tablet strength available to consumers is 25 milligrams (mg). For most adults, the recommended therapeutic dosage range for allergy relief is 25 mg to 50 mg, taken every four to six hours. Taking two 25 mg tablets simultaneously results in a 50 mg dose, placing the intake at the higher end of the standard therapeutic range.
Diphenhydramine is quickly absorbed after ingestion, with peak plasma concentration typically occurring between 1.5 and 3 hours. Taking 50 mg at once results in a faster and higher rise to its maximum concentration compared to a single 25 mg tablet.
Diphenhydramine readily crosses the blood-brain barrier. Once in the central nervous system, it blocks H1 receptors, which is responsible for both the anti-allergy effect and the strong sedative properties. Furthermore, the compound has potent anticholinergic properties because it also blocks muscarinic acetylcholine receptors.
Acute Physiological Effects
The 50 mg dose, while within the therapeutic window, heightens the expected physiological effects due to the increased drug concentration. The most immediate effect is significantly increased sedation and drowsiness. This central nervous system depression causes marked sleepiness and cognitive impairment, making tasks requiring focus, such as driving or operating machinery, unsafe.
The strong anticholinergic properties of diphenhydramine lead to a predictable set of peripheral symptoms. One common effect is dry mouth (xerostomia), which occurs when the muscarinic receptors responsible for stimulating saliva production are blocked. Blurry vision (mydriasis) is another common anticholinergic effect resulting from the drug interfering with the muscles that control the pupil.
Other side effects include dizziness and difficulty with coordination. In sensitive individuals, a 50 mg dose may also cause mild difficulty or hesitancy when urinating, known as urinary retention.
Recognizing Excessive Dosing
While a 50 mg dose is typically not considered an overdose, it can cause symptoms that border on toxicity in some individuals. Excessive dosing is marked by a severe escalation of central nervous system and cardiovascular symptoms. Early signs of toxicity include severe confusion, agitation, or profound disorientation.
A particularly concerning sign is the onset of visual hallucinations or delirium. Physiologically, the heart rate may become rapid, a condition known as tachycardia. Other symptoms of severe anticholinergic toxicity include a high body temperature (hyperthermia) and the potential for seizures.
If any of these severe symptoms—such as delirium, hallucinations, or a racing heart—occur after taking diphenhydramine, immediate medical attention is necessary. Contacting poison control or emergency services is the appropriate action. Symptoms of toxicity can be dose-dependent, and even smaller amounts can trigger severe reactions in individuals with underlying health conditions or sensitivities.