Does Alcohol Really Cause Dehydration?

• Alcohol’s effect on hydration is minimal. While it can lead to a temporary increase in urination after your first drink, it doesn’t continuously deplete water from your body with each additional drink.
• Dehydration is not the main issue. After a night of drinking, hydration loss is minor (about one glass of water), and electrolyte imbalances are usually not a concern.
• Poor sleep, gut irritation, and the toxic byproduct acetaldehyde—not dehydration—are the main culprits behind that rough morning-after feeling.

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What’s the first piece of advice you received when you started drinking alcohol? For many of us, it was the recommendation to “drink a glass of water for every drink” because alcohol supposedly dehydrates you.

But is alcohol really going to significantly dehydrate you? Turns out, the answer is no. What’s even more surprising, though, is that scientists have actually known this since 1942.

But how can that be? It's not immediately intuitive, we grow up believing otherwise, and countless sources (i.e., hydration brands and products, health blogs, and even some experts) still point to “the dehydrating effects of alcohol.” To understand alcohol and dehydration, we have to do what any good scientist would do: look at the primary research.

The research on alcohol and dehydration

This belief likely comes from the fact that alcohol is a diuretic, meaning it makes you pee. That’s true, but only up to a point. Decades of studies, including research from as far back as the 1940s show that alcohol doesn’t actually cause significant dehydration (Eggleton, 1942).

A closer look at the data explains why. In this study measuring urine output after drinking, there’s a clear pattern. When participants had a single alcoholic drink, they experienced a brief spike in urine flow—one drink, one extra trip to the bathroom. However, after that initial spike, urine output returned to normal. In other words, your first alcoholic drink will make you pee more than if you were just drinking water, but for the rest of the night any peeing you’re doing is pretty much exactly the same as if you were drinking water.

But what happens when someone has multiple drinks over several hours? If alcohol continuously dehydrated the body, you’d expect to see repeated spikes in urine flow with each drink. Well, the author of this same paper did that experiment, and this is what they saw.

After the first drink, urine output remained stable. Even with sustained high blood alcohol concentration from multiple drinks, the body didn’t continue to lose excess fluid beyond what would happen with water.

These results have been confirmed in multiple studies across different populations, types of alcohol, and varying levels of consumption. The takeaway? While alcohol does increase urine output after your first drink, it doesn’t lead to ongoing dehydration throughout the night the way many people assume.

What about electrolytes?

So, alcohol doesn’t make you dehydrated. But what about the electrolyte imbalances we’ve all been warned about? The authors of another study examined the biochemical effects of a night of drinking. At the end of the study—the morning after drinking—all the participants had some form of next-day discomfort such as headaches, nausea, dry mouth, or generally felt unwell. However, they weren’t severely dehydrated. Even after considering fluid loss over 14 hours, they were probably only short by about a cup or so of water (200-300 ml). On top of that, their blood sugar levels were normal, their electrolytes were balanced, and their liver function was fine (Gill et al., 1982).

Shockingly, the science is unequivocal and clear—and has been so for decades. The typical night of drinking does not create the effects of dehydration, electrolyte imbalance, and vitamin deficiency that we grew up believing it did.

Alcohol-related effects on your body

So, if alcohol isn’t causing significant dehydration, what is it doing instead?

We all know what alcohol is, but it’s important to note that alcohol (referring to the molecule ethanol specifically here, not alcoholic beverages generally) is a known carcinogen. In high concentrations, it’s harmful to your body—it kills cells and disrupts essential processes. There’s a reason alcohol is used as a disinfectant: it’s great at killing living things!

But at less than high concentrations, here are some general effects alcohol has on your body, aside from intoxication:

1. Alcohol affects GABA and glutamate signaling in your brain, which negatively affects the quality of your sleep, causing grogginess and daytime sleepiness (Colrain et al., 2018).
2. Alcohol irritates gastrointestinal (GI) tissues and causes inflammation, which can result in discomfort and other GI distress (Bishehsari et al., 2017).
3. Alcohol alters and disrupts the balance of your microbiome— the massively complex and ever-changing community of microbes that live in your gut—potentially causing bloating, gut and systemic inflammation, and other GI distress (Engen et al., 2015 ; Mutlu et al., 2012).
4. Alcohol inhibits antidiuretic hormone, vasopressin, which leads to the small spike in urination discussed above. However, more interestingly, this impact on vasopressin causes a pendulum swing that creates wider hormonal imbalances. Alcohol’s impact on vasopressin could be responsible for all kinds of effects as your endocrine system rebalances, including feelings of anxiety, your tolerance for alcohol, and your cravings for alcohol (Harper et al., 2019).

Acetaldehyde-related effects

Acetaldehyde is a metabolic byproduct of alcohol consumption, a known carcinogen, and a highly toxic molecule (Na & Lee, 2017). The good news is that you’re exposed to far less acetaldehyde than alcohol when you drink. The bad news is that it’s much more harmful to your body.

Here are some of the general effects of acetaldehyde exposure:

1. Acetaldehyde causes vasodilation (widening of your blood vessels), which is why your face might flush red and why you might feel hot if you drink a lot (Guivernau, 1987).
2. Acetaldehyde causes cell death and DNA damage, which can result in a host of downstream biological impacts (Mizumoto et al., 2017 ;Tsuruta et al., 2020).
3. Acetaldehyde binds to receptors in your brain, creates oxidative stress, and generally wreaks havoc throughout your body, resulting in some unpleasant symptoms if your body is exposed to enough, such as nausea, vomiting, tremors, dizziness, diarrhea, and malaise (Eriksson, 2001).

Quick tips for staying hydrated when drinking alcohol

Drink water between alcoholic drinks. Why it helps: drinking water between alcoholic beverages slows your drinking pace, supports blood volume, and can reduce overall alcohol intake. A common guide is one glass of water per alcoholic drink.

Hydrate before you start drinking. Why it helps: starting the night well-hydrated improves your body's ability to handle any fluid loss. Dehydration before drinking may have an additive effect with alcohol on impairments like cognitive performance or mood.

Summing things up: does alcohol cause dehydration?

Alcohol’s impact on the body is complex, affecting everything from sleep and digestion to inflammation and endocrine function. While dehydration isn’t the core issue many believe it to be, drinking water while consuming alcohol is still a good idea—not to combat dehydration, but to help pace your intake and support your liver and kidneys as they process alcohol. Instead of relying on rehydration aids or electrolyte powders, it’s more useful to understand the role of chemicals like acetaldehyde, the main culprit behind many of alcohol’s negative after-effects. Learn more about how your body breaks down alcohol.

This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice.