To paraphrase the old adage about beer, you never buy a cup of coffee ― you only rent it. As you wait in the line for the loo of your favourite coffee house, you might wonder why this particular drink has such a powerful effect on your bladder.

Check out what the experts have to say about that journey from first delicious sip to mad dash to wee, and every stop in between.  

Let’s start with your first sip

Everything you swallow goes straight to your stomach. Depending on how much is already in there — if this is your first sip of the day or if you’ve just eaten a big meal — it can hang out in the stomach anywhere from five minutes to upward of an hour. 

Liquids are quicker to digest than solid food, so your body gets to work on an empty-stomach cup of coffee right away, sending it through your intestines, kidneys and on the way to your bladder. And this is where caffeinated coffee is different from some other beverages, according toLisa Anderson, associate professor of integrative biology and physiology at the University of Minnesota. 

“The small intestine uses osmosis to absorb the liquid, using energy to pull in sodium and other electrolytes so the liquid follows those particles,” Anderson told HuffPost. “With caffeinated coffee, the caffeine, polyphenols and other coffee molecules are all lipid-soluble, which means they pass easily across the lining of the gastrointestinal tract.” 

We’ve finally come to the moment of truth, when it’s time to talk about your new word for the day, “micturition,” which Anderson told us is a science-person word for peeing.

“Once the fluid has made its way to the bladder, sensors there begin to notice that the bladder is filling up,” Anderson said. “That sets off signals to the micturition centre (see, there it is) in your brain stem, which sits by other centres that tell you things like when you’re thirsty, hungry or need to vomit.” 

Coffee gets the process moving faster because the caffeine it contains provides an extra boost of stimulation to the detrusor muscle, a smooth muscle in the bladder’s wall. (It stays relaxed so the bladder can store up urine, and it contracts when it’s go time.) With your brain and your bladder working together, you suddenly ask the person sitting next to you in the coffee shop, “Would you watch my stuff for a minute while I go to the bathroom?” 

This fast-track process goes even faster if you have an overactive bladder, Anderson said, citing studies of people with that disorder. Drinking eight ounces of water with caffeine made them go to the bathroom much sooner than when they were drinking just eight ounces of plain water. 

One more fun fact about the size of your bladder: That short line to the men’s room isn’t only about the patriarchy. It turns out that men “can hold more fluid in their bladders, and hold it for longer,” Anderson said.

Caffeine adds to the feeling of urgency to wee, compared to drinking the same amount of plain water

While anything you drink is eventually going to need to be eliminated, it’s the caffeine in coffee that, um, expedites the process. “Caffeine is a bladder irritant, and when the bladder is irritated, it starts to contract,” said Dr. Rena Malik, a urologist and assistant professor of surgery and director of female pelvic medicine and reconstructive surgery at the University of Maryland School of Medicine. “That contraction is what gives you that sense of what we urologists call ‘urgency,’ which is the sudden desire to go to the bathroom that you can’t delay.”

Malik noted that studies have shown that while caffeine does increase urgency, those who consume quite a lot of it — in drinks or food — may build up some resistance. In her practice, she sees a wide variety in each person’s body. “I have some patients tell me that drinking even just a cup of coffee makes their morning miserable, and others can drink quite a lot and not suffer any ill effects.”

Drink eight ounces, wee 12?

While coffee is a fluid, caffeine is a diuretic, which means that your kidneys may be pulling more fluid from your system than you’ve just consumed, without hydrating you at all. So, if you feel like you’re weeing more than you just drank, you may be entirely correct. And you need to get some plain old water into your system, stat. 

The more caffeine you drink, the worse it can get. “The diuretic properties of caffeine kick in at higher levels of consumption,” Anderson said. “You usually need to ingest more than 400 mg of caffeine for it to have a significant impact on your body’s fluid balance.” That’s roughly theamount of caffeine in four cups of brewed coffee, which is also the recommended limit for daily consumption.

When to worry that caffeine is dehydrating you

Malik said a quick way to tell if you’re dehydrated from all that coffee is to check the colour of your urine. “If it’s really dark yellow, you need more fluids in your system,” she said. “If it’s almost clear, you’re drinking too much. But if your urine is a lemonade-yellow colour, your fluid levels are most likely in a good balance.”

If you’d like to cut down on the frequency of your bathroom breaks, you don’t have to go cold turkey on the caffeine, Malik said. “It’s OK to take it slowly, so if, for example, you drink two cups of coffee every morning, try having one regular and one decaf and see what happens.” 

Finally, if you feel as if most of your life is spent in a toilet stall, Malik urges you to consult a urologist. “People might think, ‘This is just the way I’ve been my whole life,’ but you don’t have to suffer. A urologist can help you look at all your options, including lifestyle changes, medications and procedures.”