Pretty much everyone ingests caffeine in some way. The FDA says that more than 80% of American adults drink it on a daily basis, whether through coffee, tea, energy drinks or soda. Joseph Stromberg of the Smithsonian says people forget that caffeine is actually an addictive drug simply through virtue of the fact that it's everywhere, despite being "the world's most popular psychoactive" substance.
Scientists established that caffeine is chemically addictive back in 1994. But in May 2013, the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders officially included caffeine withdrawal as a mental disorder.
Here's how it works. Caffeine is both water and fat-soluble, meaning it can actually get through the blood-brain barrier and stimulate the brain directly. But it also shares similarities to adenosine, a body nucleoside that scientists believe tells the body it's tired. Caffeine can "cut in line" to enter an adenosine receptor, negating adenosine and actually tricking the brain into improving your perception of wakefulness.
Image Credit: Beautiful Brainiacs
"A side effect to caffeine's clever move here is that dopamine becomes more effective (it can improve motor skills and even arousal), and with all of the adenosine floating around the brain, unable to enter the receptor it was bound for, adrenal glands begin to secrete adrenaline — yet another stimulant," wrote Rob Williams of HotHardware.
Image adapted from animation by Alex Wong
Caffeine is mildly addictive, and regular use can cause some physical dependence. But as is obvious through casual observation, caffeine doesn't cause the same kind of dependencies as other stimulants — the most a caffiene consumer is likely to notice is a few bad days of headaches, fatigue, anxiety, or irritability, and only if they're a two-cups-or-more drinker.
Most experts discard the idea that caffeine can be seriously addictive in the vast majority of people. Likewise, it's not very deadly. Sure, 100 cups of coffee in four hours could kill you, but you'd likely vomit or have other physical reactions long before reaching the lethal cup.
However, it's still possible to develop a caffeine problem. In reaction to caffeine's adenosine-blocking effect, the brain simply makes more adenosine. Over time, users need more and more caffeine just to reach the same effect. Thus regularly drinking coffee actually changes your brain.
This image was taken from a study on methods of studying caffeine's effect on the brain, and should not be interpreted as "caffeine's effect on the brain."
Once you take a sip of coffee, it takes roughly 10 minutes for the caffeine concentration in your blood to reach half saturation, enough to have an effect. Forty-five minutes is enough for the caffeine to take its full effect. Depending on how much caffeine was ingested and how fast or slow the body's ability to break it down, effects could last three to five hours.
This awesome video from Reactions shows how three metabolites found in caffeine (theobromine, paraxanthine and theophylline) also add to caffeine's rejuvenating effect:
Finally, caffeine has actually been linked to an extensive array of positive health effects. Moderate coffee drinking — around three to four 5-ounce cups or a single Venti from Starbucks — has been linked to reduced risk of Type 2 diabetes as well as skin, prostate and oral cancer. A 2012 experiment demonstrated that caffeine may help fight off dementia. Another study in humans found that older adults with mild cognitive impairment who had higher levels of caffeine in their blood were significantly less likely to have progressed to full-blown dementia than the caffeine-free comparison group.