What happens when you drink caffeine while dehydrated?

Updated 7/29/2019.

Have you ever wondered what happens when you use coffee or energy drinks to quench your thirst? Have you ever noticed sometimes an energy drink won’t affect you the same way? Or that, on some days, it feels like your standard cup of coffee just isn’t working? Thes questions are all related through the 5 Levels of Fatigue.

Doesn’t caffeine make you dehydrated?

Whether a caffeinated drink is good or bad depends on what’s drink as well as who you are and what you’re doing. If you’re dehydrated, for example, a cup of coffee with it’s ~100 mg caffeine is not going to make you dehydrated – in fact, it might even count TOWARDS your liquid intake for the day.

As we’ve discussed in another post, it’s myth caffeine is a diuretic (something that increases the amount of urine your body makes). Caffeine is only a diuretic if:

  • You don’t drink caffeine regularly or haven’t had it in awhile
  • It’s more than 250 mg caffeine (a cup of coffee has ~100 mg, a can of Monster Energy has 160 mg caffeine, a Grande brewed coffee from Starbucks has ~320 mg caffeine)

The problem with dehydration and fatigue

I’m a food scientist who researches caffeine and energy drinks. In my expert opinion, the biggest problem with drinking coffee or energy drinks to quench your thirst is that dehydration causes fatigue.


I discussed this Fatigue-Dehydration-Caffeine connection in my book:

Here’s a book excerpt from that discussion:

“Minimal dehydration (1-2% of body weight in fluids) can slow down metabolism and make you feel thirsty and slightly fatigued

Chapter One: The ABCs of Caffeine Safety: C for Consumption Specifics

One of the reasons I developed the 5 Levels of Fatigue is because some people consume caffeine when their dehydration makes them tired.

This is “Fatigue Level 1”.

I urge everyone to consume a cup of water before reaching for caffeine, just in case. Sometimes, water (or the short walk to get the water and the subsequent trip to the bathroom) is enough to wake you up. This trick doesn’t work all the time, but it helps cut down on total caffeine consumption. If you want to avoid caffeine toxicity, tolerance, or dependence, you need to be strategic about when and how you consume it.

The 5 Levels of Fatigue works like this: An energy drink is like your favorite song. What works for YOU might not work for me, and what works for you on a Monday morning may not be your go-to for a Friday night. With the 5 Levels of Fatigue, you can learn how to calculate what you need and when, whether it’s water, a nap, caffeine, or envigorating human interaction.

Love this info? Want to learn more?

I’ve researched the science and safety behind energy drinks and their ingredients since 2003. This book is the culmination of my research:

3 thoughts on “What happens when you drink caffeine while dehydrated?

  • This article didn’t really answer anything. What happens to your body if you consume caffeine while you are already dehydrated? Does the caffeine not affect you as much, does it affect you more, do the effects take longer to wear off because your body doesn’t have enough spare water to flush the caffeine out?

    • Thank you for your comment and excellent observations. To answer your questions, when you are dehydrated, the caffeine will not feel as strong. While energy drinks do count towards that “8 oz of water per day” we’ve all been told, being dehydrated slows down metabolism and makes you perceive less energy. “Perceived Energy” is how we measure the effectiveness of an energy drink when we’re not measuring objective stats like reation time or time to exhaustion. Im short, the caffeine will metabolize and leave your body as quickly as when you’re fully hydrated (no significant difference there) but you will feel significantly less “boosted” when you have an energy drink while dehydrated vs when you’re hydrated. I hope this helps!
      Thank you for taking the time to review this article and leave a comment.

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