Opioids are a class of drugs that relieve pain by binding to specific receptors in the brain and spinal cord, reducing the intensity of pain signals sent through the body. The category includes natural compounds derived from the opium poppy plant, semi-synthetic drugs modified from those natural compounds, and fully synthetic versions created entirely in a laboratory. Some are prescription medications used after surgery or for cancer pain. Others, like heroin, are illegal.
Natural Opioids
Natural opioids come directly from the opium poppy plant. The two most well-known are morphine and codeine. Morphine is one of the oldest and most widely used pain relievers in medicine, commonly given in hospitals for severe pain after surgery or trauma. It also serves as the baseline for comparing the strength of other opioids. Codeine is milder and has historically been used for pain relief and cough suppression, often combined with acetaminophen in prescription formulations.
Semi-Synthetic Opioids
Semi-synthetic opioids start with a natural opioid molecule that is then chemically altered in a lab. This group includes some of the most commonly prescribed painkillers:
- Oxycodone (brand name OxyContin): about 1.5 times as potent as morphine when taken orally
- Hydrocodone (found in Vicodin): roughly equal in strength to morphine
- Hydromorphone (brand name Dilaudid): about four times as potent as oral morphine
- Oxymorphone (brand name Opana): another high-potency option used for severe pain
Heroin also falls into this category. It is made from morphine but is classified as an illegal substance with no accepted medical use in the United States. It is typically sold as a white or brownish powder, sometimes mixed with sugars, starch, or powdered milk. A form called “black tar” heroin is sticky or hard and dark-colored due to crude processing methods.
Fully Synthetic Opioids
Synthetic opioids are built entirely in a laboratory with no plant-derived starting material. The most significant one today is fentanyl, which is roughly 100 times more potent than morphine. Fentanyl has legitimate medical uses, particularly for severe post-surgical pain, and is available as patches, lozenges, and injectable forms. But illegally manufactured fentanyl has become the leading driver of overdose deaths in the U.S. It is often pressed into counterfeit prescription pills or mixed into other drugs, sometimes without the user’s knowledge.
Other synthetic opioids include tramadol (a lower-potency prescription painkiller), methadone (used both for pain and to treat opioid use disorder), and tapentadol. Carfentanil, a fentanyl analog designed for use in large animals, has also appeared in the illicit drug supply and is roughly 100 times more potent than fentanyl itself.
Opioids You Might Not Recognize
A few over-the-counter medications are technically opioids, even though most people don’t think of them that way. Loperamide, the active ingredient in the anti-diarrhea medication Imodium, is a synthetic opioid that acts on the same receptors in the gut. At normal doses it doesn’t cross into the brain in meaningful amounts, so it relieves diarrhea without causing a high or pain relief. The FDA originally classified it as a controlled substance before moving it to over-the-counter status. At very high doses or when combined with certain other drugs, however, loperamide can produce opioid-like effects including respiratory depression.
How Opioids Work in the Body
All opioids share a basic mechanism. They attach to proteins called opioid receptors, found mostly in the brain, spinal cord, and gut. The most important of these for pain relief is called the mu receptor. When an opioid binds to it, it triggers a chain reaction inside the nerve cell that ultimately makes the cell less excitable. On the sending side of a nerve connection, this stops pain-signaling chemicals from being released. On the receiving side, it quiets the nerve cell directly. The net effect is that pain signals get dampened before they reach conscious awareness.
This same mechanism also explains the side effects. Opioids slow breathing because mu receptors exist in the brain’s breathing center. They cause constipation because mu receptors line the digestive tract. And they produce euphoria because mu receptors sit in the brain’s reward pathways, which is what makes them addictive.
Common Side Effects
The most frequent side effects of opioids are drowsiness, mental fog, nausea, and constipation. Constipation is so predictable that it affects the majority of people taking opioids regularly and, unlike other side effects, doesn’t improve with continued use. Slowed breathing is the most dangerous effect and the direct cause of overdose deaths. The risk increases at higher doses and when opioids are combined with alcohol, sedatives, or muscle relaxants.
With repeated use, the body adapts to opioids in two important ways. Tolerance means you need progressively higher doses to get the same pain relief. Physical dependence means your body adjusts to having the drug present, so stopping suddenly causes withdrawal symptoms like muscle aches, anxiety, sweating, and insomnia. Neither tolerance nor dependence is the same as addiction, though both raise the risk. Addiction, now called opioid use disorder, involves compulsive use despite harmful consequences and affects a meaningful fraction of people who take opioids long-term.
Medical Uses Beyond Pain
Pain relief is the primary reason opioids are prescribed, particularly for acute pain after surgery or injury, cancer-related pain, and palliative care. But opioids serve other medical purposes as well. Codeine and certain synthetic compounds are used as cough suppressants. Loperamide treats diarrhea. Methadone and buprenorphine are cornerstones of treatment for opioid use disorder, where they reduce cravings and prevent withdrawal without producing a significant high at stable doses.
For chronic non-cancer pain conditions like back pain, arthritis, or nerve pain, opioids remain controversial. Current CDC guidelines recommend caution at any dosage for long-term use and advise clinicians and patients to carefully weigh benefits against risks before exceeding 50 morphine milligram equivalents per day, a threshold where the dangers begin to outweigh the additional pain relief for most people.
Drugs That Block Opioids
Opioid antagonists are the opposite of opioids. They bind to the same receptors but don’t activate them, effectively blocking or reversing opioid effects. Naloxone (brand name Narcan) is the emergency overdose reversal drug, now available over the counter as a nasal spray. It works within minutes by knocking opioid molecules off the receptors, restoring normal breathing. Its effects wear off in 30 to 90 minutes, which is why a person who has been revived may still need emergency care.
Naltrexone works on the same principle but lasts much longer. It is taken daily as a pill or monthly as an injection to help people with opioid or alcohol use disorder maintain recovery. Because it blocks all opioid effects, it must only be started after someone has fully detoxified, otherwise it triggers immediate, severe withdrawal.