Opioids include a wide range of drugs, from prescription painkillers like oxycodone and hydrocodone to illicit substances like heroin and illegally manufactured fentanyl. They all work by binding to the same receptors in your brain and body to reduce pain, but they vary enormously in strength, legal status, and how they’re used. Here’s a breakdown of the major examples and how they differ.
Natural Opioids
Natural opioids, sometimes called opiates, come directly from the opium poppy plant. The two primary examples are morphine and codeine. Morphine is the benchmark against which all other opioids are measured for strength. It’s used in hospitals for moderate to severe pain and remains one of the most widely recognized painkillers in the world.
Codeine is significantly weaker than morphine and is typically used for mild to moderate pain or as a cough suppressant. It’s often combined with acetaminophen in prescription pain products or with other ingredients in cough and cold medicines. Low-dose codeine cough preparations (containing no more than 200 milligrams per 100 milliliters) are classified as Schedule V controlled substances, the least restrictive category. Higher-dose codeine products fall under Schedule III.
Semi-Synthetic Opioids
Semi-synthetic opioids are chemically modified versions of natural opiates, engineered in a lab to change their potency, duration, or how they’re absorbed. This category includes some of the most commonly prescribed painkillers in the United States:
- Hydrocodone (brand name Vicodin) is one of the most frequently prescribed opioids in the country, used for moderate to moderately severe pain.
- Oxycodone (OxyContin, Percocet) is roughly 1.5 times as potent as morphine when taken by mouth and is used for moderate to severe pain.
- Hydromorphone (Dilaudid) is about 5 times as potent as oral morphine, making it a stronger option for severe pain.
- Oxymorphone (Opana) is another high-potency option in this class.
Heroin also belongs to this group. It’s made by chemically processing morphine and has no accepted medical use in the United States. Heroin is produced primarily from opium poppies grown in Southeast and Southwest Asia, Mexico, and Colombia.
Fully Synthetic Opioids
Synthetic opioids are built entirely in a laboratory with no plant-derived starting material. They include both prescription medications and substances found on the illicit drug supply.
Fentanyl is the most well-known synthetic opioid. In its prescription form (brand names Sublimaze, Duragesic), it’s used for severe pain, often delivered through a skin patch that’s worn for 72 hours at a time, or through lozenges and nasal sprays. Fentanyl is 50 to 100 times more potent than morphine. Illegally manufactured fentanyl, which is chemically identical but produced outside pharmaceutical oversight, has become the leading driver of opioid overdose deaths.
Tramadol is on the opposite end of the potency spectrum. It’s a weaker synthetic opioid approved for pain severe enough to require an opioid but not severe enough to warrant something stronger. Methadone, another synthetic opioid, serves a dual role: it’s prescribed both for pain management and as a treatment for opioid addiction. All of these, along with hydrocodone, oxycodone, and fentanyl, are classified as Schedule II controlled substances by the DEA, meaning they have accepted medical uses but carry a high potential for misuse.
Fentanyl Analogues and Illicit Variants
Beyond pharmaceutical fentanyl, a growing number of fentanyl analogues have appeared in the illicit drug supply. These are chemicals with structures similar to fentanyl but slightly altered, which can make them harder to detect with standard drug testing. The most commonly identified analogues include carfentanil, furanylfentanyl, and acetylfentanyl.
Carfentanil is the most dangerous of these. Originally developed to sedate large animals like elephants, it’s estimated to be 10,000 times more potent than morphine. A dose invisible to the naked eye can be lethal. Acetylfentanyl and furanylfentanyl are both less potent than fentanyl itself, but still far stronger than most traditional opioids and capable of causing fatal overdoses.
Opioids Used to Treat Addiction
Some opioids are used not to manage pain but to treat opioid use disorder itself. Buprenorphine (sold under the brand name Suboxone, among others) is a partial agonist, meaning it activates the same brain receptors as other opioids but only to a limited degree. It relieves cravings and withdrawal symptoms without producing the same euphoria as stronger opioids. It also has a built-in “ceiling effect”: after a certain dose, taking more doesn’t increase its effects, which lowers the risk of overdose.
Buprenorphine products are often combined with naloxone, an opioid antagonist that blocks opioid receptors. When the combination tablet is taken as directed (dissolved under the tongue), the naloxone has little effect. But if someone tries to inject or snort it, the naloxone activates and blocks the opioid high, discouraging misuse. Buprenorphine is classified as a Schedule III substance.
Naltrexone is another antagonist used in addiction treatment. It fully blocks opioid receptors, preventing any opioid from producing its effects. Unlike buprenorphine, naltrexone is not itself an opioid, so it doesn’t relieve cravings in the same way. If someone takes naltrexone while buprenorphine or another opioid is still in their system, it will trigger immediate withdrawal symptoms.
How Opioid Strength Is Compared
Doctors compare opioid strength using a unit called morphine milligram equivalents (MME). Oral morphine is the baseline at 1 MME per milligram. Oral oxycodone is 1.5 MME per milligram, meaning a 10 mg oxycodone tablet delivers roughly the same effect as 15 mg of morphine. Oral hydromorphone comes in at 5 MME per milligram, and intravenous fentanyl reaches 300 MME per milligram, illustrating just how wide the potency gap is across this drug class.
Current CDC guidelines recommend that doctors start opioid-naive patients at the lowest effective dose, typically 20 to 30 MME per day. Before increasing beyond 50 MME per day, clinicians are advised to carefully weigh the benefits against the risks, since higher doses tend to bring diminishing pain relief alongside increasing danger.
How Opioids Are Taken
Opioids come in a surprising variety of forms depending on the specific drug and the type of pain being treated. The most common is a simple oral tablet or capsule, which is how hydrocodone, oxycodone, and most codeine products are taken. Morphine is available as both immediate-release and extended-release oral formulations.
Fentanyl, because of its extreme potency, is available in forms that deliver very small, controlled amounts. The transdermal patch (Duragesic) is applied to a hairless area on the chest, back, upper arm, or waist and slowly releases the drug through the skin over three days. Fentanyl also comes as buccal tablets (dissolved against the cheek), lozenges, nasal sprays, and oral films. In hospital settings, fentanyl and morphine are frequently given intravenously for acute or post-surgical pain. Buprenorphine for addiction treatment is taken as a sublingual tablet or film dissolved under the tongue.