Opioid agonists are substances that activate opioid receptors in the central and peripheral nervous systems. They have several uses, from pain management to treating opioid use disorder.

You have opioid receptors throughout your nervous system. Substances that activate these receptors are known as opioid agonists or partial agonists. Opioid agonists include what people typically refer to as opioids, such as oxycodone, morphine, and heroin.

Opioids are often misunderstood. While many people associate them with the ongoing overdose epidemic, they play a vital role in the medical world for pain management. Plus, your body actually makes its own form of opioids called endogenous opioids that play a role in several bodily processes, from sensory perception to gastrointestinal function.

Here’s a closer look at how opioid agonists work, why they’re used, and how they compare to partial agonists.

Your body has three primary opioid receptor types: mu, delta, and kappa. These receptors are found on the surface of cells in the central and peripheral nervous systems. They respond to your body’s natural endogenous opioids and exogenous opioids, or those introduced into the body.

When an opioid agonist comes along, the opioid receptors activate and cue the cell to perform a function.

As opioid receptors activate, they cause a cascade of processes in your body.

For example, mu receptors are most affected by exogenous opioids. When activated, they produce feelings of euphoria, pain relief, and respiratory depression.

Opioid agonists vary in their function. Some have a broad effect, while others target specific opioid receptor sites. They may affect one or multiple receptors.

Opioid agonists can activate opioid receptors linked to:

  • pain relief
  • reduction of oxidative injury to neurons
  • respiratory depression
  • balancing of positive and negative ions in the body
  • euphoria
  • sedation
  • protective cardiac preconditioning

Common opioid agonists include:

Opioid agonists, however, are not limited to medications or external substances. Your body’s endogenous opioids include peptides such as:

  • endorphins
  • dynorphin
  • enkephalins

So, while opioid drugs are opioid agonists, not all opioid agonists are drugs.

Opioid agonists are primarily used in pain management and anesthesia. They’re indicated for surgical pain, cancer pain, and traumatic pain, particularly when non-opioid pain management isn’t effective.

Research notes opioid agonist medications are unique in their ability to relieve physical and emotional pain simultaneously by blocking pain signals and generating feel-good chemicals.

In addition to medical pain management, opioid agonists may also be used for:

  • local anesthesia
  • cough suppression
  • diarrhea control
  • pulmonary edema in myocardial ischemia
  • relief of shivering related to antifungal therapy

The role of opioid agonists in opioid use disorder

Opioid agonists have a role in the treatment of opioid use disorder.

While it may seem counterintuitive to treat opioid dependence with opioid agonists, it’s a successful option because of the varied effects of specific opioid agonists in the body.

Methadone, for example, is an opioid agonist used to treat symptoms of withdrawal in opioid use disorder. It fulfills the body’s opioid cravings by activating opioid receptors.

But unlike other opioids, it doesn’t cause euphoria and has a slower effect on the brain. It’s also sometimes prescribed for pain relief.

Buprenorphine, a partial agonist, is also used to treat opioid use disorder. It’s typically used alongside the opioid antagonist naloxone to help manage mild to moderate withdrawal symptoms.

Opioid agonists activate receptor sites and cause a cellular response. Opioid antagonists have the opposite effect. They bind to the opioid receptor but have no effect. They remain in the site, blocking it from being activated by agonists.

Common opioid antagonists include naloxone and naltrexone.

Agonists can also be classified as “full” or “partial.” A full opioid agonist draws the maximum opioid response from the receptor site. A partial agonist will also activate a receptor site but to a lesser degree.

Partial opioid agonists often have both agonist and antagonist properties. They may be referred to as “mixed agonist-antagonists.”

Medications like tramadol and buprenorphine are examples of partial agonists that also have antagonist properties. Depending on their dose, they activate certain opioid receptor sites while blocking others.

Opioid agonists are substances that bind to opioid receptor sites, cueing cells to behave in a certain way.

When opioid receptors are activated, they kick off the processes that lead to a sense of euphoria, pain reduction, and the suppression of functions like breathing and gut motility.

Your body makes its own version of opioid agonists, known as endogenous opioids, but you can also introduce external opioids into your system. These exogenous opioids include drugs like morphine, tramadol, hydrocodone, and heroin.