Understanding Diphenhydramine
Diphenhydramine is a first-generation antihistamine commonly known by brand names like Benadryl, Nytol, and Unisom [1.3.1, 1.3.6]. It is widely available over-the-counter and is primarily used to treat allergy symptoms such as sneezing, runny nose, itchy or watery eyes, and hives [1.3.2, 1.3.4]. Because it crosses the blood-brain barrier and causes drowsiness, it is also frequently used as a short-term sleep aid and for motion sickness [1.3.1, 1.2.7]. As a first-generation antihistamine, its sedative effects are a well-known side effect, along with dry mouth, dizziness, and blurred vision [1.3.2, 1.3.6]. These effects occur because the medication blocks histamine H1 receptors in the central nervous system (CNS) [1.3.7].
How Alcohol Affects the Body
Alcohol is also a central nervous system (CNS) depressant [1.4.1]. It slows down brain activity, which can impair judgment, coordination, and reaction time [1.4.4]. Immediate effects include slurred speech, memory problems, and reduced inhibitions [1.4.7]. Over time, chronic and excessive alcohol use can lead to severe health complications, including liver disease, heart problems, neurological damage, and an increased risk of cancer [1.4.2]. Like diphenhydramine, alcohol's depressant effects are a primary reason for its potential for harm, especially when consumed in large quantities or combined with other substances.
The Compounded Danger: Mixing Alcohol and Diphenhydramine
The primary danger of mixing alcohol and diphenhydramine lies in their synergistic effect as CNS depressants [1.2.2, 1.2.7]. When taken together, they don't just add to each other's effects; they multiply them. This can lead to a dangerous level of sedation and respiratory depression, where breathing can become dangerously slow or even stop [1.2.3, 1.5.7].
The National Highway Traffic Safety Administration (NHTSA) has stated that diphenhydramine can impair a driver's alertness more than alcohol can [1.2.2]. Combining them significantly worsens this impairment, making activities like driving or operating machinery life-threatening [1.2.6].
Key Risks of Combining Alcohol and Diphenhydramine
- Intensified Sedation and Drowsiness: The combined effect can lead to extreme drowsiness, confusion, and dizziness far beyond what either substance would cause alone [1.2.3]. This can result in falls, accidents, and injuries [1.2.2].
- Impaired Cognitive Function: The mixture significantly impairs judgment, coordination, and reaction time [1.5.3]. It also affects memory and learning, as diphenhydramine blocks acetylcholine, a neurotransmitter crucial for these functions [1.2.6].
- Increased Risk of Overdose: Combining these two depressants dramatically increases the risk of a fatal overdose [1.2.3]. Symptoms of a combined overdose include extreme drowsiness, confusion, hallucinations, seizures, difficulty breathing, and loss of consciousness [1.5.3, 1.2.4].
- Cardiovascular Effects: Diphenhydramine can increase heart rate and blood pressure. Alcohol can exacerbate these effects, posing a risk to individuals with pre-existing heart conditions [1.2.3].
- Dehydration: Both substances are dehydrating, and when combined, they increase the risk of headaches, dizziness, and can worsen a hangover [1.2.6].
Comparison of Effects
| Effect | Diphenhydramine Alone | Alcohol Alone | Combined |
|---|---|---|---|
| CNS Impact | Depressant, causes drowsiness [1.3.8] | Depressant, slows brain activity [1.4.1] | Synergistic and severe CNS depression [1.2.2] |
| Coordination | Impaired [1.3.1] | Impaired [1.4.2] | Severely impaired coordination and motor skills [1.2.3] |
| Drowsiness | Significant [1.3.6] | Can cause drowsiness [1.4.2] | Extreme, potentially leading to unconsciousness [1.2.2, 1.2.3] |
| Overdose Risk | Possible, with symptoms like seizures, hallucinations [1.3.3] | Possible, with symptoms like slow breathing, vomiting [1.5.4] | Significantly increased risk of fatal overdose [1.5.3] |
| Heart Rate | Can increase heart rate [1.2.3] | Can raise blood pressure [1.4.1] | Exacerbated cardiovascular effects [1.2.3] |
Special Risk Groups
Certain populations are at an even higher risk of severe adverse effects when mixing alcohol and diphenhydramine.
- Older Adults: The aging process slows the body's ability to metabolize both alcohol and drugs [1.2.6]. This means the substances stay in their system longer, increasing the risk of a harmful interaction. Older adults are also more susceptible to side effects like dizziness and loss of coordination, raising the risk of falls [1.3.4, 1.2.2].
- Females: Due to differences in body composition and water content, females generally experience the effects of alcohol more intensely than males, making the combination even riskier [1.2.6].
- Individuals on Other Medications: Diphenhydramine can interact with other medications that cause drowsiness, such as benzodiazepines, opioids, and other sleep aids, further compounding the depressant effects [1.2.4].
What to Do in an Emergency
If you suspect someone has overdosed on a combination of alcohol and diphenhydramine, it is a medical emergency. Call 911 immediately.
Signs of an overdose can include:
- Extreme drowsiness or inability to stay awake [1.5.3]
- Confusion or delirium [1.2.4]
- Slowed, irregular, or difficult breathing [1.5.3, 1.5.4]
- Hallucinations [1.2.4]
- Seizures [1.5.3]
- Rapid heartbeat [1.5.1]
- Clammy skin [1.5.4]
Do not try to make the person vomit. If they are unconscious, roll them onto their side to prevent choking in case they do vomit [1.5.4].
Conclusion: A Combination to Avoid
It is never safe to drink alcohol while taking diphenhydramine. Both are central nervous system depressants, and their combined use leads to a dangerous amplification of side effects, including severe drowsiness, impaired motor control, and cognitive deficits. This interaction significantly increases the risk of accidents, injuries, and a life-threatening overdose. The safest course of action is to completely avoid alcohol when using any medication containing diphenhydramine [1.2.6]. Always read medication labels and consult with a healthcare professional about potential interactions [1.2.5].
For more information on medication interactions with alcohol, you can visit the National Institute on Alcohol Abuse and Alcoholism (NIAAA).
