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A few years ago, a friend of mine who shall remain nameless threw a 30th-birthday party with a very specific theme: Drink like you’re turning 21.

I wasn’t that far from 21 at the time, and so I didn’t fully fathom the extent of the suffering she was about to bring upon herself. Later, someone else told me the birthday girl had been hung-over for two days straight — and it was horrifying knowledge. Learning about the existence of the multi-day hangover in your early 20s is a bit like finding out the exact day you’re going to die: There’s really nothing you can do except stare into the inevitable future, knowing that it holds something deeply unpleasant.

Unfortunately, it’s one of the few certainties we have in the science of hangovers. There’s still plenty that alcohol researchers don’t know for sure — like, um, why they happen — but there are enough pieces of anecdotal and scientific evidence to suggest that hangovers really do get worse with age. (The lone bright side: You’re not imagining things.)

One of the major reasons for this is that you simply become less efficient at processing your drinks. Each beer or margarita or Jell-O shot you force on your poor body takes about an hour to break down. Doing so is a multi-step process: First, a liver enzyme called alcohol dehydrogenase transforms the alcohol you’ve ingested into a compound called acetaldehyde. Next, another enzyme called aldehyde dehydrogenase breaks that down into acetate, which then becomes carbon dioxide and water.

When you’re 21, this process acts as a fairly well-oiled machine. But over time, our levels of the necessary enzymes decrease, meaning acetaldehyde — which is a highly toxic, nasty chemical — spends more time hanging out in your system, causing headaches, mouth dryness, nausea, and a host of other symptoms.

The overall composition of your body is also changing: As they grow older, people may also build up more body fat, which leaves them more susceptible to alcohol’s effects. Fat doesn’t absorb alcohol, meaning someone who has more of it will have less space for booze to dilute — it’s the reason women, who generally have more body fat than men, also tend to have lower tolerance. The body also loses water with age — you have more water in you at 20 than you do at 40 — which, again, means the booze stays more concentrated in your system.

On top of all these changes, there’s the added, overarching fact that you’re just not as good at recovering — from anything — as you used to be. As Time has explained:

If you have kids, you know a toddler’s bruised knee or scraped knuckle miraculously heals in a day or two, while a cut on your finger might linger for a week or more. Likewise, you may remember easily recovering from heavy exercise during your teens or twenties, but now a long workout may leave you sore for days. The National Institute on Aging refers to this as “immunosenescence,” or the gradual weakening of your immune system as you age. It’s not that your body doesn’t heal; in many cases, it just doesn’t heal quite as quickly, research suggests.

Things get rougher in the later part of middle age, too. As The Wall Street Journal has pointed out, older people are also more likely to be on medications for chronic illness, increasing the chance that they’ll have a drug in their system that messes with the metabolization of alcohol. Alcohol also speeds up the natural cognitive decline people experience in their 50s and 60s, and this decline, in turn, worsens the effects of each drink:

Booze basically enhances normal age-related cognitive decline. Neurons lose speed. Specifically, the insulating myelin sheaths around the axons of neurons—the parts responsible for transmitting information to other neurons—get smaller. As people age, “neurons are not as efficient. So you impair them with a little bit of alcohol, they are that much more inefficient,” says Dr. Oslin. “Somebody who goes to a cocktail party at 65 can have one or two drinks and be really impaired.”

Still, the body isn’t the only thing to blame for those increasingly miserable post-drinking ruts — lifestyle plays a role, too. It’s a lot easier to cope with a pounding headache when the only thing on your Sunday agenda is eating Seamless in bed and groaning softly to yourself. But throw some grown-up responsibility into the mix — say, a couple of screaming kids — and that same headache becomes another beast entirely.

Meanwhile, a 2013 survey from the U.K. group Redemption, which promotes alcohol-free bars, found that the age of Peak Hangover is 29 — not because of any specific physical factors, but more an unfortunate confluence of behavior and biology. At 29, people are often still holding on to the drinking habits of their younger years, even as their mornings-after become increasingly more dire. If a 29-year-old and a 45-year-old drank the same amount, in other words, the 45-year old may have the more intense hangover; the difference is that the 45-year-old is less likely to put themselves in that state in the first place.

So, you know, maybe the best way to avoid a killer hangover is to stare down your own mortality, drinking with the caution of someone who knows they’re aging with every sip. Also, water. Be kind to yourself. Biology certainly won’t be.