Many over-the-counter (OTC) medications used for insomnia have primary effects on the H1 receptor. At this receptor, these medications serve as an antagonist; hence the name antihistamine. The H1 receptor is primarily implicated in allergic reactions, so blockade of this receptor decreases the allergic response. Histamine receptors in the central nervous system also have a role in wakefulness; blocking them causes sedation. Studies report that nearly 25 percent of patients with sleep disturbances use OTC sleep aids such as antihistamines, and 5 percent use them at least several nights a week.
Routine use of OTC antihistamines such as diphenhydramine (Benadryl) and doxylamine (Unisom SleepTabs and Nyquil) for insomnia is controversial. I know that on inpatient psychiatric hospitals, these medications are frequently used for sleep because 1) they are relatively safe and 2) they are not addictive. However, the evidence supporting the use of these medications for long-term treatment of insomnia is lacking. Additionally, many OTC antihistamines (especially Benadryl and doxylamine) have effects at cholinergic receptors leading to uncomfortable side effects such as dry mouth, blurred vision, constipation, urinary retention, and more importantly cognitive impairment and possibly delirium 1.
Recent literature has explored the long term implications of use of anticholinergics (one of the more common being OTC antihistamines). A large prospective cohort study demonstrated that greater cumulative use of anticholinergic medicine in people >65 is associated with an increased risk of dementia 2. To quote from the study:
Higher cumulative anticholinergic medication use is associated with an increased risk for dementia. Efforts to increase awareness among health professionals and older adults about this potential medication-related risk are important to minimize anticholinergic use over time 2.
In cases where use of a medication is controversial; its important to be an informed consumer. Benadryl or Doxylamine? What dose is effective? Is Tylenol PM different than Benadryl? Is Nyquil? Is it okay to take these medications if you’re >65?
With regard to insomnia, diphenhydramine has been studied more thoroughly and is noted with short-term use to lead to improvements on self-reported sleep efficiency and the Insomnia Severity Index, and a decrease in participant-reported number of awakenings 3. Unfortunately, most studies fail to observe significant changes on the polysomnogram for sleep-onset, sleep efficiency, and total sleep time 3. Thus, although you may feel like you’re sleeping better, the data doesn’t show this. Furthermore, long-term use of OTC antihistamines may be futile. In fact, studies demonstrate that diphenhydramine may lose its sleep-promoting effects after just 3 days. Furthermore, when dosed at 50mg versus 25mg (i.e. 2 Benadryl vs 1) participants had significant psychomotor impairment and a decreased level of wakefulness the next morning 3.
The studies on doxylamine are few and far between. Similar to diphenhydramine, studies show improvement with self-reported sleep but not with the polysomnogram 3. The main drawback to doxylamine? It has a longer half-life than Benadryl, and thus is likely to cause more morning impairment than a comparable dose of Benadryl.
|Brand Name||Benadryl, ZZQuil, Tylenol PM, All Unisom products except SleepTabs||Nyquil, Unisom Sleep Tabs|
|Half-Life||3-9 hours (usually reported as 8)||10 hours|
My recommendation? If you’re having insomnia and you want a quick fix, go with 25mg of diphenhydramine each night for a short period of time (I’d recommend no more than 1 week). If you’re over 65, be extremely judicious with your use of OTC antihistamines. Likely due to their anticholinergic effects, long-term use is associated with an increased risk of dementia. Short-term use is probably okay, but if you continue to have sleepdisturbances after 1 week of use, ask your doctor for a better, more evidence based, long term solution.