The most commonly used psychoactive drug in the world — caffeine — is also one of the most common performance-enhancing drugs used in sports.

The reason? It works, provided you carefully calculate the right dose and don’t use too much of it in day-to-day life.

“If you can tolerate it, it seems to be the upper end of what you can have to improve performance,” exercise physiologist Matthew Ganio told The Atlantic. If dosed correctly (and if it isn’t just returning a caffeine addict to a baseline level), it provides just enough of a boost that athletes see notable performance gains.

If you’ve ever had a killer migraine, you’ve likely tried Excedrin, an over-the-counter medication marketed specifically for these types of rare, severe headaches. What you might not know is that in addition to traditional pain relieving ingredients like ibuprofen and acetaminophen, Excedrin contains caffeine.

There’s some evidence that caffeine, when combined with certain pain-relieving medications like acetaminophen (the main active ingredient in Tylenol) and aspirin, helps those medications take effect quicker, last longer, and increases their effects.

For example, a 2007 study of 24 people who took either caffeine and the painkiller acetaminophen, either drug alone, or a placebo found that those who’d taken the combination of (as opposed to either one alone) saw a stronger decrease in pain symptoms that also tended to last longer.

It’s natural to grow increasingly tired throughout the day — our brains naturally produce more and more of a molecule called adenosine from the time we wake up until the time we go to sleep. Scientists think this helps us get to bed at night.

Caffeine hijacks this natural process by mimicking adenosine in the brain. It latches onto the receptors designed for adenosine, pushing them out of the way. As a result, we’re left feeling more alert and awake.

A cup of coffee likely blunts your appetite for a brief time, but there’s little to no evidence that making caffeine a regular habit can keep hunger pangs at bay or help with weight loss.

Most studies looking at caffeine’s effect on appetite have either been too small or only done in animals, making it hard to say their effects would apply to people more broadly.

Caffeine has been shown to improve certain types of memory in some (but not all) studies, especially the ability to remember lists of words and straightforward information. Some research shows that it helps those memories “stick” in the brain as well, making it easier to recall that information at a later time.

One recent study indicates that extroverts get more of a working-memory boost from caffeine than introverts. This may explain why some studies find a more significant effect than others. Stephen Braun, the author of “Buzz: The Science and Lore of Alcohol and Caffeine”, explains that individual’s reactions to caffeine vary greatly: while one person might thrive on a high level of caffeine, it’ll make another person unable to get anything done.

This enhancement, however, seems to be strongest for people who aren’t already hooked on caffeine in the first place, and too much caffeine can actually lead to a decrease in performance.

As a central nervous system stimulant, caffeine doesn’t just boost alertness, it can also improve your mood and is even associated with a reduced risk of depression — especially when consumed in the form of coffee.

Even though too much of any stimulant can make people anxious and irritable, a mild dose has been shown to boost mood. This is due to the same adenosine-blocking effect that makes you feel alert. By blocking adenosine’s relaxing effects, caffeine lets dopamine and glutamine, another natural stimulant produced by your brain, run wild, making you more alert, less bored, and providing a mood boost.

Interestingly, a number of studies have found a connection between caffeine consumption and a reduced risk of depression (and even a lower risk of suicide). However, at least one of these studies specifically found this connection with caffeinated coffee but not tea, though others found the same effect for tea as well.

Eventually, adenosine wisens up to caffeine’s act, though, and makes new receptors for the sleep-inducing molecule to start latching on again.

This is why your morning cup of coffee can suddenly turn into two — the more receptors you have, the more caffeine you need to plug them up.

Some research suggests that for some people, caffeine’s perceived benefits aren’t really benefits at all.

For some, these studies suggest, all of the benefits of caffeine — from better mood to improved memory and attention span — may simply be the result of a new dose of caffeine temporarily reversing the effects of longer-term withdrawal from the drug.

In other words, when someone who’s hooked coffee stops drinking coffee, she feels tired and is less attentive. When she starts drinking again, her performance increases — but only because her brain and body had already become addicted to caffeine.

Ever wonder if that late afternoon cup of coffee (or that evening espresso) is going to keep you up at night?

It takes about 5 or 6 hours before half of the caffeine you’ve ingested wears off — so a couple cups of coffee at 4 p.m. could leave you still feeling the effects of a cup of coffee by 10 p.m.

If that’s enough to keep you up, plan accordingly.

Adrenaline is the hormone behind the “fight or flight” response. It tells us either to stay and face a threatening situation or flee the scene. Unfortunately, while a burst of adrenaline can be helpful for ditching someone chasing after us or defending ourselves in a fight, the aggressive hormone does no good when we’re trying to handle a more delicate situation like negotiating in a meeting or responding to a text.

Caffeine excites our brain cells, which tells our hormone control center the pituitary gland that there’s an emergency. The pituitary tells the adrenal glands (located above the kidneys) to flood the body with adrenaline.

In this excited state, we’re great at running and fighting but we also tend to be more irritable, anxious, and far more emotionally-charged.

While a warm cup of coffee might seem soothing if you’re stressed out, it may have negative effects as well. Not only can that adrenaline burst increase anxiety levels, caffeine also raises acid levels in your stomach. This can lead to heartburn and can be especially bad if you suffer from ulcers.

One of the things that’s most rarely discussed with regard to caffeine is that it is, in fact, a drug. It has psychoactive effects, changing the way we feel and interact with the world around us.

It’s the most commonly used psychoactive drug in the entire world, which is probably why we don’t think about it as a drug. Yet think of how many of us can’t — or won’t — get through a day without it (this writer included).

Harvard neuroscientist Charles Czeisler thinks that caffeine, combined with electricity, allowed humans to escape natural patterns of sleep and wakefulness, breaking them free from the cycle of the sun, essentially enabling the “great transformation of human economic endeavor from the farm to the factory,” according to a look at this miracle drug in National Geographic. It enables the modern world.

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