Many of us have experienced, seen, or at least heard of an odd medical phenomenon.

• Some people who take a dose of Nyquil don’t get a good night’s sleep. They’re up all night.
• Some people who take anti-anxiety medications end up having panic attacks.
• Some people taking prescription opioids find that their pain actually gets worse.

In the medical world, those counter-intuitive experiences are called “paradoxical reactions.” The exact reasons for them aren’t clear, but generally speaking, they seem be related to defects or deficiencies in the way the brain responds to signals from the central nervous system.

Paradoxical reactions are most often seen in patients taking the sedatives known as benzodiazepines, but they can occur when taking almost any medication or drug.

And they happen with caffeine, too.

A cup of coffee most often wakes you up in the morning, rejuvenates you in the afternoon, or – if you’re not a decaf person – keeps you up at night. But the caffeine in coffee can produce other, undesired effects.

Caffeine is regularly used or prescribed to help treat headaches. It’s even an ingredient in many OTC (over-the-counter) pain relievers. But – in a paradoxical reaction – caffeine can also make headaches worse. That effect is seen most often in those who suffer with migraine headaches.

And, as you probably know, headaches can also result from caffeine withdrawal.

There’s a complicated relationship between caffeine and headaches. Let’s try to make it a little less mysterious.

Most of us know caffeine as a rather potent stimulant. Strictly speaking, it’s a psychoactive drug – one which alters the way we feel and think – and it’s the most commonly-used psychoactive drug in the world. (Interestingly, caffeine and cocaine act on the same part of the brain.)

We experience the familiar effects of caffeine for several reasons.

First of all, caffeine somewhat blocks the brain’s ability to stay on an even keel. It does that by mimicking the structure of an important biochemical, adenosine. Adenosine is responsible for balancing other chemical levels in the brain; but because caffeine is so similar to adenosine, it is able to enter and block the brain’s adenosine receptors. In other words, caffeine prevents adenosine from moderating the way we feel and act.

That allows hormones and neurotransmitters like dopamine (the “feel-good hormone”), serotonin (the “happy chemical”) and adrenaline (the “fight or flight” hormone), to run wild, explaining the surge of energy we feel after a Starbucks latte.

And a recent study shows that caffeine even helps the brain process information more quickly, well beyond the initial jolt that the stimulant provides.

There’s another important effect of caffeine to consider. Throughout the body it acts as a vasodilator, expanding blood vessels and increasing blood flow. In the brain, however, it only briefly expands the blood vessels – after that it, as it interacts with adenosine receptors, caffeine actually acts as a vasoconstrictor and reduces cerebral blood flow.

Remember that last point. Understanding caffeine headaches depends on it.

There are two major types of headaches.

Tension-type headaches are the most common variety, causing pain in the head, neck and/or behind the eyes. For about two-thirds of Americans tension headaches occur just occasionally, but they’re a chronic problem (15 or more days per month) for about three percent. Painful, throbbing migraine headaches are somewhat less-common, affecting around 43% of women and 18% of men.

Migraines aren’t understood very well, although genetics appears to play a major role. And there are many other possible causes of headaches. Both stress and migraine headaches can be triggered by a variety of other factors including stress, physical ailments, colds and infections, environmental issues (like strong odors or smoke), foods and allergens, even bad posture. For some people, headaches are also triggered by certain foods or medications.

What causes headache pain? It’s primarily due to changes that occur in the brain after exposure to headache triggers. Once triggers are detected, chemical messages are sent throughout the brain, leading to increased blood flow. That, in turn, causes an expansion of blood vessels around the brain. And as the vessels expand, they place pressure on adjoining nerves which send signals to the brain’s pain centers – producing the head pain commonly associated with headaches.

That helps explain how and why many headache treatments work.