Opioids slot into opioid receptors and activate them. This sends signals to your brain to relieve pain and promote pleasure.

Both endogenous opioids, which your body naturally produces, and exogenous opioids, which are opioid medications or substances, can activate opioid receptors.

The receptors are part of your body’s endogenous opioid system, which deals with pain, reward, and addiction. They’re present on nerve cells, mainly in your central nervous system (CNS) and gut.

There are three main types of opioid receptors: mu, delta, and kappa. Although they’re similar in some ways, they’re distributed differently throughout your body and can produce different effects.

Mu opioid receptors occur throughout your CNS, particularly in areas that deal with sensory perception.

These were the first opioid receptors to be discovered. Those who found them named them after their sensitivity to morphine.

Later on, however, experts found that most opioid drugs could activate mu receptors, and that this could lead to dependence on these drugs in some cases. Side effects like breathing difficulties and constipation can also happen.

However, these are also the main receptors for the body’s own opioids, sending chemical signals that reduce certain brain neuron activity. This leads to pain relief and the stimulation of the reward system, which can result in dopamine production and feelings of euphoria.

There are fewer delta opioid receptors throughout your body. They tend to accumulate in the forebrain.

When they bind with some of the body’s own opioids, called enkephalins, they can reduce anxiety as well as pain. This suggests that the receptors play a role in mood regulation.

One study found that targeting delta opioid receptors could reduce pain for longer and may result in fewer side effects when treating pain.

Delta receptors may have other roles, too, involving cardiovascular regulation and the movement of food through the gastrointestinal system. However, these roles need further research.

Kappa opioid receptors bind with natural opioids in the body called dynorphins.

They’re the only opioid receptor that doesn’t cause respiratory depression. They can also have an anti-reward, dysphoric effect, which is the opposite effect of mu receptors.

So, while they do provide pain relief like the other opioid receptors, their side effects are generally less severe. You’re also less likely to become dependent on a drug that binds with a kappa receptor.

This all means that they may be a good candidate for newer forms of pain treatment. Research is ongoing in this area.

However, one review notes that withdrawal from opioids can cause stress in the body, boosting the function of kappa receptors and potentially making relapse more likely due to a continued dysphoric mood.

Opioid drugs, which doctors typically prescribe for pain relief, can activate opioid receptors just like the body’s natural opioids can. But there’s research to suggest that they work in a slightly different way to endogenous opioids.

While naturally produced opioids activate receptors on the surface and inside of nerve cells, opioid drugs may also activate extra parts of nerve cells. And it’s this additional activation that may lead to problematic side effects.

With long-term use, opioid drugs like oxycodone and morphine can also lead to addiction due to the euphoria that people feel when taking them.

Opioid addiction is fueled by the opioid receptors turning off certain nerve cells in the midbrain, allowing dopamine to take over.

Dopamine makes the opioid drug feel rewarding, leading people to want to take more. When a person no longer takes these drugs, the body switches the other way, causing feelings of dysphoria and anxiety and leading to more opioid cravings.

Three primary types of opioid receptors — mu, delta, and kappa — exist in the body.

Opioids that the body naturally produces and opioids that you take in the form of a medication or substance can activate these receptors.

All opioid receptors have a positive effect on pain. But some come with unwanted side effects, including addiction, particularly if the receptors are activated over and over again in the long term.

Lauren Sharkey is a United Kingdom-based journalist and author specializing in women’s issues. When she isn’t trying to discover a way to banish migraine, she can be found uncovering the answers to your lurking health questions. She has also written a book profiling young female activists across the globe and is currently building a community of such resisters. Catch her on Twitter.