How the Diphenhydramine in Benadryl Makes Allergy Season Better and Pupils Tiny

Let’s talk about Benadryl. If you’ve ever swelled up into a red-faced, itching balloon after going balls out on a shellfish-laden seafood platter or nuzzling an especially hirsute cat, the anti-allergy drug, known technically as diphenhydramine, can be a lifesaver. Sometimes you don’t know you’re allergic. Sometimes you don’t care. And who can blame you? Crab legs are delicious and cats are fucking cute. Fortunately, diphenhydramine acts reliably and responsibly when your brain doesn’t.

You know you’ve encountered an allergen once the swelling, itching, watery eyes, and sneezing set in. Assuming your reaction isn’t so severe that you go into anaphylaxis — that is, swelling so rapid that your airways get choked off — Benadryl makes the ensuing symptoms more manageable. What it does, essentially, is help your body relax after allergens cause it to lose its chill.

To understand how Benadryl, an antihistamine, deflates your red, rapidly ballooning face, it helps to understand how allergic reactions work in the first place. Histamine, one of the body’s many tiny signaling molecules, is released by white blood cells in your immune system after they encounter things your body is pre programmed not to like. These allergens, depending on your genes and how much exposure you’ve had to the world’s toxins, can range from the usual — things like hay fever-inducing pollen and poison ivy — to the deeply weird, like tanned leather or water (seriously).

Once released, the tiny histamine molecules then bind to histamine receptors lining the surface of your muscles, kicking off what’s called the inflammatory response: As they bind and your blood vessels suddenly dilate, fluid — containing immune cells ready to throw down with offending pathogens — floods into the surrounding tissues. Hence, swelling. And, because your poor flesh isn’t used to being pumped full of liquid, pain. In order to wage war, these immune cells are equipped with seriously savage physiological weaponry, like chemicals that raise the local temperature to smoke out enemies. Histamine also activates the nerves that tell your brain to scratch an itch. The inflammatory response is a hot mess, but it’s necessary for keeping invaders out.

Still, it can be disproportionately severe, especially when the offending molecules aren’t actually that offensive. Viruses need to be stamped out immediately; dandelion dust, not so much. But the immune system is a prickly, hypochondriacal thing that isn’t afraid of pulling out all the stops when it’s threatened.

This is where diphenhydramine can help. Benadryl’s main ingredient is classified as an “inverse agonist of the histamine H1 receptor,” which is a complicated way of saying its job is to box histamine out of its slot on the receptor, preventing it from kicking off the inflammation process altogether. Taking an anti-allergy pill floods your bloodstream with diphenhydramine molecules, which prevent histamine from binding. Rapid relief ensues, allowing your hot, swollen body to return to its normal size.

Of course, taking Benadryl has side effects, some of which can be pretty useful. Sure, the pasty mouth, racing heart, dilated pupils, constipation, and occasional inability to hold an erection can suck, but its sedative effects make it a pretty decent sleep aid, as anyone who’s popped an antihistamine before bed knows.

So go on, wear leather pants at your next prairie picnic and keep on sucking the sweet meat out of crustacean limbs. Benadryl is your friend, as long as you don’t overdo it: At extremely high doses, diphenhydramine has been known to cause hallucinations and delirium, and not exactly the fun kind.