Just about everyone has had a bad night — or morning after — caused by a little too much drinking. That’s because alcohol is, more or less, a poison. And yet, some people get sick no matter how careful they are or end up suffering from serious reactions far outside the norm.

What’s up with those folks?

Well, science shows there are three major reasons alcohol causes people real problems at a biological level.

Alcohol Flush Reaction

When some people drink, they turn bright red. Called alcohol flush reaction by doctors and ‘Asian glow’ by sophomores, it’s a serious scolding from evolution that’s demanding you pass on that next drink.

See, alcohol is churned out of your body through your liver with a one-two punch. First, an enzyme in the liver turns alcohol into acetaldehyde. Acetaldehyde is practically a poison, which is treated like any other substance and eventually peed out.

But for many — including Asians and Ashkenazi Jews, among others — the acetaldehyde dehydrogenase, or ALDH2, gene is hopelessly mutated. So acetaldehyde remains in its nasty acetaldehyde state, causing liver damage, ulcers, and cancers of the esophagus and stomach. The mutation also means you turn bright red and may cause you to feel like your heartbeat’s rising or your stomach’s churning.

Instead of covering up the blush and continuing to drink, it’s important to remember the blush is just one outward sign of a deeper internal problem.

See also: Science Explains… How Life Experience Determines Alcohol Preference

An Allergy to Other Ingredients in Your Preferred Beverage

When people have a true allergic reaction to an alcohol beverage, it’s probably not from the alcohol at all. Hives, trouble breathing, stomach cramps, runny or stuffy noses, and even collapse are all signs that the common sulfites, grains, or histamines in alcohol are making you sick.

Sulfites occur naturally when you ferment beer and wine. They stop the bad bacteria from growing, allowing the good stuff to ferment. Some manufacturers love the positive effects of sulfites so much, they add extra artificially. But for people who have pre-existing asthma, sulfite can exacerbate other breathing problems. Some manufacturers are developing sulfite-free formulations, but for most people, the best and easiest thing is to stop drinking entirely. Histamines are another organic byproduct of brewing that can irritate your body but can’t be easily eliminated from alcoholic beverages.

But if neither of these are your problem, there are plenty of other suspects. Grains are clearly the essential ingredient in beer and a lot of hard alcohol is made of wheat. This is clearly a problem for those with celiac disease. As with sulfite-free wine, there are some gluten free manufacturers and, thanks to the recent gluten free craze, the number of options is growing and supposedly getting tastier.

Ultimately, alcohol manufacturers don’t have to list every single ingredient blended into a drink. So if you’re still getting sick, near-invisible spices, fruits, or other things thrown in the mix may be the source of your problems. If you’re insistent that you want to keep drinking, you’ll have to do a trial and error method to discover what exactly is bothering you — and what beverages might be ok. But if your symptoms are severe, it’s probably best to talk with a doctor before hitting the bottle again.

See also: Synthetic Alcohol Promises to End Hangovers for Good

PSA: Everyone’s Limit is Different

Even someone without an ALDH2 gene mutation or any sulfite, histamine, or gluten intolerance can still get really crazy sick from drinking. In these cases, it might be a (re)learning your limit.

A lot of people want to offer a shortcut for how much alcohol someone can drink while staying under the legal or social limit. Just think how many people you’ve heard swear that “one drink an hour” is still sober. But everyone’s alcohol tolerance is fundamentally different.

Women typically can’t handle as much alcohol as men. People who are heavier can likely drink more than someone thinner. Health conditions like asthma can be exacerbated by drinking. Drinking on an empty stomach clearly makes people tipsier faster. Hard alcohol like vodka will definitely get you drunk before beer does. And younger people tend to be able to drink more with fewer ill effects than older people. (That’s why your hangovers have felt like they’re getting worse as you age. They are.)

But it goes beyond that — even the way you’re drinking can change how your body copes with alcohol. Seriously. Drinking caffeine and alcohol simultaneously doesn’t make you less drunk. It just makes you feel less drunk so you can drink more — and more dangerously, too. And a lot of other things also change your perception of your own drinking. One 2012 study showed that the when alcohol is poured into a curved glass (as opposed to a straight one), drinking speed increased by 60 percent. That’s a big change in consumption from a simple bend in a line.

In the end, just because there are a certain number of built-in side effects to drinking doesn’t mean you should continue throwing it back if you feel something’s off. If you’re quickly sick, anything about your breathing seems labored, get hives, or your face swells or turns red, it’s time to head to the clinic.

Photos via Flickr / cutglassdecanter