Opioids are a broad class of drugs that relieve pain by binding to specific receptors in the brain and spinal cord. The category includes everything from natural compounds extracted from the opium poppy to entirely lab-made chemicals like fentanyl. Some are prescribed for pain after surgery or injury, others are used to treat coughs or addiction, and some exist only as street drugs. Here’s a clear breakdown of which drugs fall into this category and how they differ.
How Opioids Work in the Body
All opioids, regardless of their origin, target the same set of receptors in the brain. The most important of these is the mu receptor, which is responsible for both pain relief and the feeling of euphoria that makes these drugs addictive. When an opioid binds to this receptor, it blocks pain signals traveling from the body to the brain. At the same time, it triggers a release of dopamine, the brain’s reward chemical, which creates a sense of pleasure and reinforces the desire to use the drug again.
This same receptor also controls breathing rate, which is why opioid overdoses can be fatal. High doses suppress the brain’s drive to breathe. Other effects tied to these receptors include constipation, constricted pupils, and drowsiness.
Natural Opioids (Opiates)
The oldest opioids come directly from the opium poppy plant. These are sometimes called “opiates” to distinguish them from lab-made versions. The key natural compounds are:
- Morphine: The principal active compound in opium and the benchmark against which all other opioids are measured for strength. It’s used in hospitals for severe pain, including after major surgery and heart attacks.
- Codeine: A milder relative of morphine, often combined with acetaminophen for moderate pain or used in prescription cough syrups. Low-dose codeine cough preparations are classified as Schedule V, the least restrictive controlled substance category.
- Thebaine: Not used as a painkiller itself, but serves as the raw material for manufacturing several semi-synthetic opioids, including oxycodone and buprenorphine.
Opium also contains papaverine, which relaxes smooth muscle and is used in surgery, and noscapine, which suppresses coughs. These don’t produce the classic opioid “high” but are still part of the opium family.
Semi-Synthetic Opioids
These drugs start with a natural opium compound that is then chemically modified in a lab. They are among the most commonly prescribed painkillers in the United States, and several have been central to the opioid crisis. All are classified as Schedule II controlled substances, meaning they have legitimate medical uses but carry a high potential for abuse and dependence.
- Oxycodone (sold as OxyContin, Percocet): Roughly 1.5 times as potent as morphine milligram for milligram when taken by mouth. One of the most widely prescribed and most misused opioids.
- Hydrocodone (sold as Vicodin, Norco): Similar in potency to oral morphine. Often combined with acetaminophen for moderate to severe pain.
- Hydromorphone (sold as Dilaudid): Significantly stronger than morphine, typically reserved for more severe pain.
- Oxymorphone: Another high-potency option used for severe pain that hasn’t responded to other treatments.
- Heroin: Derived from morphine, heroin is a semi-synthetic opioid with no accepted medical use in the United States. It is classified as Schedule I.
Fully Synthetic Opioids
These are designed entirely in a laboratory with no plant-derived starting material. Some are prescription medications, while others are produced illicitly.
- Fentanyl (sold as Sublimaze, Duragesic): Up to 100 times stronger than morphine and 50 times stronger than heroin. Prescribed as patches or lozenges for severe chronic pain, but illicitly manufactured fentanyl is now the leading cause of opioid overdose deaths in the U.S.
- Methadone (sold as Dolophine): A long-acting synthetic opioid used both for pain management and as a treatment for opioid addiction. Its effects last much longer than most opioids, which helps reduce cravings.
- Tramadol: A lower-potency synthetic opioid that also affects serotonin and norepinephrine levels in the brain. This dual action puts it in a category sometimes called “atypical opioids,” and it may carry a somewhat lower risk of breathing problems compared to traditional opioids like morphine or oxycodone.
- Meperidine (sold as Demerol): An older synthetic opioid once commonly used in hospitals, now less favored due to side effects.
- Carfentanil: An extremely potent synthetic opioid intended for sedating large animals. It has appeared in the illicit drug supply and is dangerous in microscopic amounts.
Atypical Opioids
A few opioids work on the mu receptor but also act on other chemical systems in the brain, giving them a different risk profile.
Buprenorphine (sold as Suboxone, Subutex) is a partial opioid agonist, meaning it activates the mu receptor only partially. This creates a “ceiling effect” where increasing the dose doesn’t keep increasing euphoria or breathing suppression. It’s classified as Schedule III and is widely used to treat opioid addiction. Tapentadol is another atypical opioid that combines mu receptor activity with norepinephrine effects, similar to tramadol but stronger.
Potency Differences Matter
Not all opioids are equally strong, and the differences are dramatic. Using oral morphine as the baseline, hydrocodone is roughly equivalent (1:1), oxycodone is about 1.5 times stronger, and fentanyl is in an entirely different league. A dose of fentanyl measured in micrograms (millionths of a gram) can match what takes milligrams of morphine. This is why accidental fentanyl exposure, even in tiny amounts, can be lethal.
These potency differences also explain why switching between opioids requires careful dose conversion. A dose that’s safe for one drug can be fatal if applied to a stronger one.
Dependence and Withdrawal
All opioids can cause physical dependence with regular use, even when taken exactly as prescribed. Dependence means the body adapts to the drug’s presence, and stopping abruptly triggers withdrawal symptoms: muscle aches, anxiety, sweating, nausea, diarrhea, and insomnia.
The timeline depends on which opioid you’re taking. Short-acting opioids like heroin or immediate-release oxycodone typically cause withdrawal symptoms within 8 to 24 hours of the last dose, with symptoms lasting 4 to 10 days. Long-acting opioids like methadone have a slower onset of withdrawal, beginning 12 to 48 hours after the last dose, but symptoms can stretch to 10 to 20 days. Withdrawal is intensely uncomfortable but rarely life-threatening on its own.
Reversing an Overdose
Naloxone (sold as Narcan) is a medication that rapidly reverses opioid overdoses by knocking opioids off the brain’s receptors. It’s available as a nasal spray that delivers 4 mg per dose and can be used by anyone, no medical training required. If someone is unresponsive and barely breathing after opioid use, a single spray into one nostril can restore breathing within minutes.
Overdoses involving fentanyl or other synthetic opioids often require multiple doses of naloxone because these drugs bind so tightly to receptors. Emergency responders may need to administer 10 mg or more in total to fully reverse the effects. Naloxone wears off faster than most opioids, so a person who responds to it can slip back into overdose and may need additional doses while waiting for emergency help.