Cite this article

NIDA. (2014, September 22). Medicines or Poisons?—Why Cannabinoids Can Both Help and Hurt You. Retrieved from

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The NIDA Blog Team
September 22 2014
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This is the final post of a 3-part series on the science of medical marijuana. Check out Part 1: What’s Wrong with “Medical Marijuana”? and Part 2: Making Medicine from Marijuana.

People who write about the health benefits of marijuana sometimes think it’s ironic that a plant containing compounds that could treat disease (like THC or CBD) is banned by the government for being unsafe. But in fact many effective, FDA-approved medicines are closely related to illegal, harmful drugs and are sometimes even made from the same sources.

That’s because there’s a fine (and sometimes fuzzy) line between chemicals that are good for you and those that can hurt or even kill you. The Greek word pharmakon, where we get pharmacy, originally meant both “medicine” and “poison.”

Speaking the Body’s Language

The opium poppy is a great example. It’s the source of a drug called morphine, part of a class of drugs called opioids. Morphine is used to make heroin, a very addictive and sometimes deadly drug. But morphine is also modified to make many effective, relatively safe pain relievers prescribed widely by doctors and dentists. In fact, these opioids are our most valuable drugs for pain relief.

Another example is cocaine, from the coca plant. It's part of a class of drugs called stimulants. Cocaine is an especially dangerous, addictive stimulant, but it's closely related to medications used to treat people with attention deficit hyperactivity disorder (ADHD) and other conditions.  Sometimes it's also used as an anesthetic.

The thing that makes a drug a drug is its ability to speak the body’s language—specifically, to interact with one of the many chemical signaling systems that cells use to talk to each other. Both heroin and cocaine are able to do that, fluently.

Same with marijuana: Its cannabinoid chemicals speak the body’s own endocannabinoid language.

Parlez-Vous Endocannabinoid?

Nerve cells use chemicals called neurotransmitters to send each other messages, and there are several different kinds of neurotransmitters. Similar chemicals in plants or in foods can interact with these neurotransmitter systems because their molecules are very similar to the ones produced naturally in the human body.

Morphine from the poppy plant is able to work in a peson's nervous system because it closely resembles the body’s own natural pain-relieving opioid chemicals—the endorphins that cause a “runner’s high.” (The “endo” in endorphin or endocannabinoid means “from inside”—that is, inside your body.)

Cocaine and related stimulants work with your own neurotransmitter dopamine, which naturally keeps you focused on rewarding activities.

And the THC in marijuana interacts with the endocannabinoid signaling system used by the body’s own cannabinoid chemicals—such as anandamide—in brain circuits that control a wide range of things including pleasure, memory, thinking, concentration, movement, coordination, and even how you perceive time. That’s why THC can interfere with these abilities when people smoke marijuana either to get high or to treat a medical condition.

The endocannabinoid system also is involved in things like appetite and pain, which is why THC has been made into an effective medication for helping treat nausea and loss of appetite in AIDS and cancer-chemotherapy patients. And it's why THC may, in the future, be prescribed for treating pain.

So, there’s nothing special about marijuana: It’s one of many plants that contain substances that can be both beneficial and harmful, depending on how they're used.

Update: Read the blog post, “How Legal Is Marijuana?