Why is some allergy medicine non-drowsy and some drowsy?


Why not just make them all non-drowsy…

In: 266

The drowsy kind works the best. The non-drowsy kind sacrifices some effectiveness in order to reduce the side effects of drowsiness.

It’s a similar dynamic with a lot of medicines. The most effective ones often have some of the most problematic side effects. Opioids are the gold standard for pain relief, there’s no other medication more effective. But they’re dangerous and addictive. An over the counter drugs like advil is safer and non-habit-forming, but it also does next-to-nothing to relieve pain.

Histamine is, among many other things, the “be awake” neurotransmitter. Block histamine in your brain and you get sleepy. The non-drowsy medications don’t cross the blood-brain barrier, so they don’t have that effect.

There’s a thriving market for stuff that makes you sleepy, so Benadryl and Unisom aren’t going anywhere. Even if they probably should, since they might be slowly giving people dementia.

Diphenhydramine (benadryl) has been on the market for a really long time, having been first approved in the 1940s. It’s also extremely cheap compared to many alternatives, but it’s extremely sedating and that’s just a characteristic nature of that compound

In contrast, some of the more modern antihistamines (Zyrtec, Allegra, Claritin, etc) were only approved for OTC use around the 2000s, with patent expiry on the name brands occuring around the end of that decade.

Many people like the fact that it makes them (or their children, more often) drowsy. It’s not seen as a downside, but a perk. Though obviously in some circumstances, non-drowsy is desirable and necessary.. so they sell both.

I used to use Cetirizin and it didn’t make me drowsy until the next day when it hit me like a truck and had to take a nap before driving home. I was certain I’d fall asleep when driving…but to answer your question based on my doctor’s response years ago:

Antiallergics can make you feel tired if they pass the blood-brain barrier and affect the nerve cells in the brain. For these cells, histamine produced by the body in very small amounts is a kind of “wake-up call” that promotes alertness. If the histamine access is blocked by a medication, the result is fatigue. Newer allergy medications cannot overcome the blood-brain barrier well.