From coca leaves to coffee beans, people use plants to produce many of the most popular drugs in the world. But whether it’s your $5 morning latté or a line of coke, you might be surprised to learn why plants bother to build the molecules behind that buzz in the first place. Strangely enough, many plant-based drugs—such as caffeine, cocaine, nicotine and morphine—are all made for the exact same reason: to fight off insects. Why exactly do humans love ingesting insect repellent so much?
Caffeine, Cocaine, Nicotine and Morphine: Pleasurable Pesticides
According to Dr. David Kennedy, who studies plants and the human brain at Northumbria University, to understand what it is about nature’s pesticides that gets us so enjoyably high, it first helps to look at the world from a plant’s perspective. “Unlike animals, plants are rooted in where they live, and can’t really get away from any threats they might need to avoid,” Kennedy says. So to keep hungry herbivores at bay, he explains, many plants can manufacture a slew of defensive chemicals.
Now some plants, like the itchy poison ivy or poison oak, use brute force chemical weapons. But others—such as opium poppies and tobacco plants—take a more delicate approach. These plants still require some animals to get close enough to help them pollinate and breed, so rather than launching a full-scale toxic offensive, they’ll merely mess with a munching bug’s mind.
To do so, these plants produce neurotoxic drugs called alkaloids, which change the balance of chemicals in a bug’s brain. At high enough levels, these drugs can kill insects (and overdose humans) but small amounts will only send them on a bad trip.
Human and Insect Brains
Oddly enough, although these alkaloids evolved to interact with insect brains, “their effects on humans are often strangely similar,” says Kennedy. “For instance, if you give cocaine to bees, it will make them dance more. If you give caffeine or other amphetamines to flies, it will wake them up and make them more aroused. And if you give morphine to insects, it’ll have the same analgesic sort of effect.”
But Kennedy explains this isn’t all that surprising. “Humans have essentially the same brain as an insect. Ours are a little more complicated, but functionally they’re both very similar,” he says. For example, in both brains many of the chemicals that the neurons use to communicate—called neurotransmitters—have the same jobs.
But the mental effect of these drugs does differ in one huge way. “Insects don’t find these drugs addictive or pleasurable, they just find them repulsive,” says Kennedy. This is because human brains have a pleasure-causing reward system which is unlike anything found in the head of a bug—and is based around a neurotransmitter called dopamine. “In humans, by total chance, these drugs just hijack that reward system,” and can flood our brains with dopamine, says Kennedy.
This dopamine rush is what causes the pleasurable effect of these drugs—which can range from a perky disposition (caffeine) to gripping euphoria (cocaine)—and is also what makes these drugs so addictive. But bugs just feel crazed or twitchy, without the pleasure.
Marijuana and Psychedelics
Not all alkaloids or insect repellents in the plant world elicit such a big wave of pleasure in humans. In fact, drugs like cocaine and caffeine are only a tiny subset, and there are plenty of similar drugs out there that will make you little more than sick.
And Kennedy says that when speaking about these addictive drugs, it’s also worth mentioning a few other chemicals that plants produce to interact with the wildlife around them: psychedelic drugs like psilocin (the active ingredient in magic mushrooms) and tetrahydrocannabinol (the active ingredient in marijuana).
Kennedy explains that these psychedelics are distinct from the addictive alkaloids—and this is because of both their chemical structure and the fact that they’re not used solely by plants as pesticides. Rather, these psychedelic drugs can have a large mix of jobs inside the plant, from fighting fungus and microbes to luring in pollinating insects. But just like the alkaloids, their insane effect on the human mind is entirely coincidental, says Kennedy.