Antihistamines & Allergies

Feb 12, 2019

By Trevor Shewfelt

It's minus 30 with a wind chill of absolute zero. I'm running late for work. I'm standing on my deck having a staring contest with my dog, Sheldon. He is below the deck staring up at me. I'm waving a cookie at him. This is usually fool proof. "Come on Sheldon. Let's go!" I've got to get him inside so I can get to work and he doesn't freeze his cute fuzzy butt off. But Sheldon is torn. He wants the cookie, but he doesn't want to drop the treasure he just dug up out of the snow. That's right ladies and gentlemen. Cute, fuzzy Sheldon has a disgusting habit. Sheldon loves turd-sicles.

Disgust is said to be the instinct you have to learn. Although it feels automatic to us to be disgusted by cockroaches, you'll find them for sale to eat in many markets in Asia. We think spaghetti is delicious, but in certain parts of Africa it is viewed as disgusting because it looks way too much like a bowl of maggots. You definitely don't need to learn to be allergic to something. It just happens. If you get a sudden itch, or running nose or watery eyes, it could be an allergy. To treat allergic reactions, one of the types of medication we often reach for are antihistamines. Oral antihistamines have been used for itchy watery eyes and runny noses since the 1940's. Antihistamines block the H1 histamine receptor inside the nose, lungs, eyes and skin.

Allergic reactions start more or less the same way. Your body comes in contact with the substance which you are allergic to, which is called the allergen. Certain cells in your immune system release a chemical messenger called histamine in response to the allergen. The histamine fits into the H1 histamine receptor like a key into a lock which activates the H1 receptor. The activated H1 receptor tells your body to start having allergy symptoms like a runny nose, or an itchy rash. The easiest and best way to avoid an allergic reaction is to identify and avoid the allergen. For example, if you are allergic to the cat at your mother-in-law's house, avoid your mother-in-law. If that horrible cologne your wife bought gives you an itchy rash, don't use it. Simple, right? Well, even if we can identify the allergen, sometimes they are hard to avoid. If you are allergic to pollens that are common in Manitoba, you will probably get a stuffy nose in the spring no matter what you do. So if we can't identify and avoid the allergen, we may need to block the H1 histamine receptor with a medication.

We block the H1 histamine receptor with antihistamines. There are two basic types of antihistamines, first generation and second generation. They both work well, but their differences are speed, side effects and price. The first generation antihistamines are have names like chlortripolon (chlorpheniramine), and benadryl (diphenhydramine). First generation antihistamines block the H1 histamine receptor, but they also block other receptors. We call them poorly selective. That leads to some of their side effects like dry mouth, because they affect other receptors. First generation antihistamines are also lipophilic which means they can get through the blood brain barrier. The fact first generation antihistamines can get into the brain and effect histamine receptors there, leads to side effects like drowsiness. In summary, they are inexpensive, work quite well, but they also often cause drowsiness, and some other side effects like dry mouth, urinary retention, and worsening glaucoma. They work fast, which is great if you have a runny nose or an itchy rash, but they don't last that long and you may have to take several pills a day.

Second generation antihistamines cause much less sedation, last longer and have far fewer side effects, but they are more expensive. Second generation antihistamines are much more likely to only block the H1 receptor. We say this makes them much more selective. So we get fewer side effects like dry mouth. Second generation antihistamines are much less lipophilic. This means they are far less likely to get across the blood brain barrier and cause problems like sedation. These are agents like reactine (cetirizine), claritin (loratadine), aerius (desloratidine) and allegra (fexofenadine). A nice benefit is one pill often lasts all day. Reactine is good for both runny noses and itchy rashes, but I have seen it make some people sleepy. Claritin only works well on runny noses, but it is much less likely to make someone sleepy. So they are more expensive, but have fewer side effects and more staying power.

As useful and common as antihistamines are, they can cause problems. They are on the BEERS list. That is a list of meds we should avoid in elderly patients. To greater or less extent, antihistamines have anticholinergic effects which means they can cause confusion, cognitive impairment, dry mouth, constipation and urinary retention. These are all effects to avoid in the elderly, especially patient who already have dementia or men who already have benign prostatic hypertrophy (BPH) or enlarged prostate. There is also concern that antihistamines might not leave the body as fast in the elderly which could make their effects last longer than we thought. The take home message should not be fear of antihistamines, but talk to your pharmacist before taking them.

On a recent episode of the podcast "Hidden Brain" they were talking about how we learn to enjoy painful things like eating hot peppers or running marathons. They called it benign masochism. They even talked about dogs. They said in one Mexican town, dogs eat food scraps with lots of hot peppers, because that was what was available, but given a choice the dogs preferred non-spiced food. In another example an owner walking his dog in Washington Square in New York was intentional sprinkling hot chili powder on dog turds in the park so his dog wouldn't eat them. It didn't work. The dog ate them anyway. Well 2958 km North West of Washington Square, this dog owner is not going to sprinkle hot chilis on dog turds. Maybe I should just get the kids to clear our yard of dog turds. Because, I'm not going to do it. I'm not a benign masochist or any other type of masochist. It's too cold out.


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