From the Guidelines
Long-term use of diphenhydramine (commonly known as Benadryl) can lead to several concerning health effects and should generally be avoided for extended periods. Regular use beyond a few weeks can cause tolerance, requiring higher doses for the same effect, and may lead to physical dependence. Chronic diphenhydramine use is associated with cognitive impairment, including memory problems and confusion, particularly in older adults 1. Research suggests a potential link between long-term anticholinergic medication use, including diphenhydramine, and increased risk of dementia. The drug can cause persistent side effects like:
- Chronic dry mouth (which may lead to dental problems)
- Constipation
- Urinary retention
- Blurred vision Some users experience daytime drowsiness, impaired coordination, and decreased alertness even with regular use. For those using diphenhydramine as a sleep aid, it typically becomes less effective over time and may disrupt normal sleep architecture, potentially worsening insomnia in the long run 2. These effects occur because diphenhydramine blocks histamine and acetylcholine receptors in the brain, affecting multiple body systems beyond just allergy symptoms. If you're currently using diphenhydramine regularly, consult with a healthcare provider about safer alternatives for your specific condition. It's also important to note that the American Academy of Sleep Medicine does not recommend the use of diphenhydramine for the treatment of chronic insomnia due to its lack of efficacy and potential for adverse effects 2.
From the FDA Drug Label
Warnings Do not use ... When using this product marked drowsiness may occur ... excitability may occur, especially in children
The FDA drug label does not answer the question.
From the Research
Long-term Effects of Diphenhydramine
The long-term effects of diphenhydramine (Benadryl) have been studied in various research papers. Some of the key findings include:
- Increased risk of cognitive decline and other adverse effects in older hospitalized patients, with a dose-response relationship 3
- Association with worsening of cognitive function, increased incidence of dementia, and increased mortality in older people due to its anticholinergic activity 4
- Significant and common side effects including sedation, impairment with decreased cognitive function, poor sleep quality, dry mouth, dizziness, and orthostatic hypotension 5
- Potential adverse central nervous system effects, including decreased rate of cognitive processing and subjective somnolence, particularly in the elderly 6
- Sedative effects that can persist well into the next day, potentially interfering with daytime performance and safety 7
Specific Effects on Cognitive Function
Research has shown that diphenhydramine can have negative effects on cognitive function, including:
- Increased risk of delirium symptoms, including inattention, disorganized speech, and altered consciousness 3
- Decreased rate of cognitive processing, as measured by the P300 event-related potential 6
- Significant performance deficits on tests of divided attention, working memory, vigilance, and speed 7
Comparison with Other Antihistamines
Studies have compared the effects of diphenhydramine with those of newer, second-generation antihistamines, such as loratadine and cetirizine. These studies have found that: