Diphenhydramine hydrochloride (Benadryl) is a first-generation antihistamine used to relieve allergy symptoms like sneezing, runny nose, and itching by blocking histamine effects. Standard over-the-counter dosing for adults is 25 milligrams (mg) per pill, with a recommended dose of 25 mg or 50 mg every four to six hours. Ingesting three pills, or 75 mg, places the dose above the standard therapeutic range for most healthy adults.
Expected Physiological Effects of 75 Milligrams
A 75 mg dose of diphenhydramine amplifies the drug’s side effects due to its ability to penetrate the blood-brain barrier and its anticholinergic properties. The most noticeable effect is pronounced central nervous system (CNS) depression, resulting in severe drowsiness and sedation. This level of sedation impairs judgment, reaction time, and the ability to perform complex tasks, such as driving or operating machinery.
The drug’s anticholinergic action, which blocks acetylcholine receptors, becomes evident at this increased dose. Common physical effects include pronounced dry mouth (xerostomia) and dry eyes. Users may also experience blurred vision and mild dilation of the pupils (mydriasis), which is characteristic of anticholinergic exposure.
Coordination will be affected, leading to unsteadiness and ataxia. Anticholinergic effects can also extend to the urinary system, potentially causing difficulty with urination or urinary retention. While typically not life-threatening for a healthy adult, 75 mg results in an exaggerated physiological response compared to the standard 50 mg dose.
Variables That Influence Individual Response
The effects of a 75 mg dose are not uniform, as biological and external factors influence how the body processes the medication. Age is a significant variable; older adults are sensitive to the anticholinergic effects of diphenhydramine. This increased sensitivity can lead to a higher risk of confusion, dizziness, and falls, making the drug generally inappropriate for this population.
Metabolism plays a substantial role, as diphenhydramine is processed in the liver by the cytochrome P450 enzyme system, notably CYP2D6. Genetic variations in this enzyme can lead to “poor metabolizers,” meaning the drug remains active in their system for longer. Conversely, “ultrarapid metabolizers” may break down the drug too quickly, sometimes leading to paradoxical effects like agitation instead of sedation.
Drug interactions present a significant risk factor, particularly when diphenhydramine is combined with other central nervous system depressants. Co-ingestion with alcohol, sedative-hypnotics, or opioid pain relievers can compound the depressive effects, increasing sedation and respiratory risk. Similarly, taking 75 mg alongside other medications that possess anticholinergic properties, such as tricyclic antidepressants or certain antipsychotics, can intensify side effects like urinary retention and rapid heartbeat.
Recognizing the Need for Medical Assistance
While 75 mg is a high dose, it is important to distinguish the expected exaggerated side effects from the symptoms of a dangerous level of toxicity. The most serious signs of toxicity involve profound changes in mental status and cardiac function.
Immediate emergency medical services (911) are required if the individual experiences seizure activity, an inability to be awakened, or signs of severe cardiovascular distress, such as a rapid or irregular heartbeat. Other critical signs include severe confusion, delirium, or hallucinations, which indicate a life-threatening anticholinergic syndrome.
If concerning symptoms appear but do not immediately require 911, the national Poison Help hotline (1-800-222-1222) should be contacted immediately. Poison control experts provide guidance on whether symptoms are manageable at home or if a trip to the emergency room is necessary. Provide them with the person’s age, weight, and the exact amount of drug taken.