How do antihistamines work?


How do antihistamines work?

There are two major classes of antihistamines, Charles Joseph Lane, M.D., a board-certified allergist based in Lynchburg, Virginia, tells SELF. First, there are the older medications, referred to as first-generation antihistamines, which include drugs like diphenhydramine (Benadryl) and chlorpheniramine. Then there are the newer, second-generation antihistamines (also sometimes called nonsedating for reasons which will become obvious), which include many of the oral allergy medications you’ll find at the drugstore, like cetirizine (Zyrtec), levocetirizine (Xyzal), fexofenadine (Allegra), and loratadine (Claritin).

All of these medications work generally in the same way: by targeting the body’s histamine-production system, which generates many of the symptoms we think of as “allergies.” Normally, when your body is exposed to an allergen that you’re sensitive to—tree pollen, cat dander, dust, etc.—it erroneously interprets that substance as a threat and sets off an immune response to combat it. This happens when the proteins in an allergen bind to IgE antibodies on the surface of mast cells, which causes the cell to release histamine, Dr. Lane explains. That histamine goes on to bind to histamine receptors, which then sets off reactions such as a runny nose, watery eyes, and other classic allergy symptoms.

Antihistamine medications bind to histamine receptors on cells throughout the body, blocking them from setting off the immune system response downstream. In that way, they can help manage allergy symptoms. Both first- and second-generation antihistamines do this by specifically blocking the H1 histamine receptor (there are other types of histamine receptors, but H1 is the one that matters most for allergic reactions). But, in addition to H1, earlier antihistamines can also cross into the brain and bind to receptors for acetylcholine, a neurotransmitter involved in muscle contraction and blood vessel dilation. That made it possible for them to also cause more wide-ranging side effects and may even come with an increased risk for dementia.

Side Effects

Older antihistamines, such as Benadryl and Atarax, have a significant amount of anticholinergic side effects, including dry mouth, drowsiness, constipation, headache, and urinary retention. Because of the side effects of these medications, they are generally considered to be too sedating for routine daytime use. Since older antihistamines can impair mental and motor functioning, they can diminish your ability to operate motor vehicles or heavy machinery.

In many states, you can be charged with driving-under-the-influence (DUI) if you operate a motor vehicle while taking medications such as Benadryl.

Newer, low-sedating antihistamines, such as Claritin and Zyrtec, tend to have fewer anticholinergic side effects. While these newer antihistamines may still cause drowsiness or a dry mouth, they haven’t been shown to impair your ability to operate a motor vehicle. Allegra is the only antihistamine that is truly considered to be non-sedating.


How do I take them?

These medicines come in a variety of forms, as mentioned above. Your doctor or pharmacist will advise you on how to take your medication, including what dose and how often. Read the leaflet that comes with your particular brand for further information.

Can these OTC meds cause false positive for PCP?

Naproxen sodium 220 mg 4x per day, Ibuprofen, Aleve, Melatonin, Allergy medication (benadryl, equate brand), eye drops for allergy relief (they have antihistamines). I took two Naproxen sodiums 10 minutes before a UA and it showed I was positive for PCP. I am freaking out because I don't use… read more

Updated 13 Mar 2017 · 1 answer

Taking antihistamines with other medicines, food or alcohol

Speak to a pharmacist or GP before taking antihistamines if you’re already taking other medicines.

There may be a risk the medicines do not mix, which could stop either from working properly or increase the risk of side effects.

Examples of medicines that could cause problems if taken with antihistamines include some types of:

Try not to drink alcohol while taking an antihistamine, particularly if it’s a type that makes you drowsy, as it can increase the chances of it making you feel sleepy.

Food and other drinks do not affect most antihistamines, but check the leaflet that comes with your medicine to make sure.

Can you take antihistamines when pregnant?

Drug companies are naturally pretty reluctant to go around testing medicines on pregnant or breast-feeding women, so there aren’t any studies to guide them. They tend to advise against to be on the safe side, although there is no evidence they cause a problem. Discuss this with your doctor who will talk you through the options. If the benefits of treatment are thought to outweigh any possible risks, the one usually advised is loratadine.

Olanzapine has an antihistamine effect. Can it safely be taken with betahistine dihydrochloride?

I have started taking Zyprexa 2.5. Would olanzapine (Zyprexa) reduce the effect of the betahistine? (I take 24 mg of betahistine dihydrochloride daily0

Updated 11 Nov 2013 · 1 answer

7. Tolerance (sort of)

If you feel like the antihistamines you’ve been taking aren't working as well as they used to, you’re not alone. Experts aren’t sure exactly what’s at the root of this issue (whether we’re really building up a true tolerance to the medication or our symptoms are just getting more severe, for instance), but it’s one that allergists see frequently, Dr. Lane says.

Luckily, if you’re using OTC medications, this problem has a pretty easy fix: Switch to a different one! In fact, Dr. Lane says that some people are able to switch to a different medication for a while and then switch back to their old standby a few months or years later if they start to have problems with the newer one. So this issue isn’t always permanent, he says.

What types of antihistamines are available?

Antihistamines are often split into two groups: first generation and second generation.

The first generation group is the older version of the medication and can cause side effects such as drowsiness.

With developments in medicine in recent years new antihistamines have been released that do not cause this side effect. These are known as second and third generation antihistamines.

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