Antihistamines are a common type of medication that helps treat allergies. They are often available over-the-counter (OTC), and some of these OTC medications can cause slight sedative effects, making you sleepy.
If you drink alcohol alongside taking OTC antihistamines, it may worsen the sedative effects, leading to increased drowsiness and impaired functioning or coordination.
This is because antihistamines and alcohol have the same effects on your central nervous system (CNS). Both substances cause the slowing down or sedation of your CNS. So if you take them at the same time, their effects combine and intensify. This can make you feel drowsier, along with other side effects.
Antihistamines and alcohol
Antihistamines help manage symptoms of allergic reactions, like seasonal allergies. They are very common medications, with many people who experience seasonal allergies in the United States receiving prescriptions for antihistamines.
Many symptoms of an allergic reaction are the result of excess levels of certain chemicals called histamines that your immune system releases when your body detects an allergen. Too much histamine causes the following symptoms:
- itching
- sneezing
- a scratchy throat
- runny nose
- watery eyes
Antihistamines work by competing with histamines for their receptors, the histamine receptors. This blocks the action of histamines and halts your allergy symptoms.
However, some types of these medications cross the blood–brain barrier. The blood–brain barrier is a term used to describe the network of blood vessels that supply blood to your CNS. They have unique properties that allow them to tightly regulate the movement of substances between your bloodstream and the brain.
Involved in this process are histamine receptors that affect the activity of your CNS, including muscle contraction. When certain types of antihistamines block these receptors, it can lead to drowsiness.
Effects of alcohol
Alcohol also has sedative effects on the CNS, as it also can cross the blood–brain barrier. This means that if you consume both alcohol and certain types of antihistamine medications at the same time, you can experience additive effects. This means the effects of the substances magnify each other.
According to the America Osteopathic College of Dermatology (AOCD), it is mainly first-generation antihistamines, or H1 antihistamines, that can cross the blood-brain barrier, meaning they are more likely to interact poorly with alcohol by depressing the CNS.
Second-generation (or H2) antihistamines are unable to cross this barrier and do not have the associated sedative effects.
Researchers note that alcohol worsens the sedative effects of H1 antihistamines, but this does not happen with H2 antihistamines. This shows that the interaction between antihistamines and alcohol involves the blood–brain barrier and whether the drugs cross it.
Side effects
H1 antihistamines are commonly associated with sleepiness and a sedative effect.
Side effects from H1 antihistamines typically only happen in higher doses. Some of these possible side effects include:
- drowsiness
- dry mouth
- dry eyes
- dizziness
- trouble sleeping
- abdominal pain
- headaches
If you consume alcohol with H1 antihistamines, you may notice the worsening of certain side effects. It’s important to remember that H2 antihistamines are unlikely to cause the same side effects that alcohol causes. Both alcohol and H1 antihistamines have an effect on your CNS
As a result, combining the H1 antihistamines and alcohol can have adverse effects on health, including:
- more extreme tiredness
- loss of concentration
- dizziness
- upset stomach
- blurry vision
- low blood pressure
Everyone’s reaction to these two substances will be different, depending on certain factors like:
- your age
- weight
- overall health
- tolerance to alcohol
- tolerance of antihistamines
- what dosage of antihistamine you take
- the amount of alcohol you consume at the time
It is important to avoid drinking alcohol if you take H1 antihistamines. If you do so, it is important to remember your dosage and keep your alcohol consumption to a minimum.
Always follow the recommended dosage instructions on medication labels and avoid consuming alcohol if the label of your medication recommends doing so.
Antihistamine examples
Only certain types of antihistamine medications cause worsening side effects if you consume them with alcohol.
H1 antihistamines block two types of receptors, the histamine and muscarinic receptors, and pass across the blood–brain barrier.
There is a high concentration of histamine receptors in your hypothalamus, a region of the brain important for controlling your state of wakefulness. As H1 antihistamines pass into your brain, they can cause drowsiness, enhanced by alcohol, which operates in the same way.
However, while H2 antihistamines block histamine receptors, they do not pass the blood–brain barrier.
This means that, unlike H1 antihistamines, H2 antihistamines do not react with alcohol in the same way as H1 antihistamines, as these medications are unable to reach the CNS.
Examples of first-generation antihistamines include:
- dimenhydrinate (Dramamine)
- diphenhydramine (Benadryl)
- doxylamine (Unisom)
Examples of second-generation antihistamines include:
- loratadine (Claritin)
- cetirizine (Zyrtec)
- fexofenadine (Allegra)
While H2 antihistamines are much less likely to cause drowsiness, it is still better to avoid taking them while drinking alcohol. This is because there is still a chance that they may reduce your alertness and slow your CNS function.
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