It can start with a pill that looks familiar. Or a powder that seems like something else. Sometimes, a person may not even realize a drug contains opioids at all.

That’s part of the danger with synthetic opioids today. They can look like other substances, and some are mixed into drugs without a person’s knowledge. But they can act very differently in the body — fast, strong, and sometimes without warning.

For people living with opioid use disorder (OUD), understanding what these drugs are and why they’re risky isn’t just information. It can be lifesaving. Today’s drug supply has changed in ways that make past experience less reliable. What once felt predictable may no longer feel that way.

Natural, Semisynthetic, and Synthetic: What’s the Difference?

Opioids are often grouped into three types:

• Natural opioids, including morphine and codeine, come directly from the poppy plant.
• Semisynthetic opioids, such as oxycodone and hydrocodone, as well as heroin, are made by chemically changing natural opioids.
• Synthetic opioids, including fentanyl and many fentanyl analogs, are fully lab-made.

This difference matters because synthetic opioids can be designed to be extremely potent. Some are much stronger than morphine.

Fentanyl, for example, is estimated to be about 50 to 100 times stronger than morphine.

That strength can be useful in medical settings, where doses are carefully measured and monitored. But it becomes extremely dangerous when production, dosing, and purity are uncontrolled. Even a tiny amount can overwhelm the body, especially if a person’s opioid tolerance (how much of the drug the body is used to) is low or has changed over time.

This can happen after a period of abstinence, when a person hasn’t used the drug for a while. Tolerance may be lower after treatment, withdrawal, a hospital stay, or time in jail.

Medical-Use vs. Illegal Synthetic Opioids

Not all synthetic opioids are illegal drugs. Some play an important role in medical care.

Doctors may use fentanyl during surgery or to treat severe pain in carefully controlled doses. In those settings, the drug is measured precisely, monitored closely, and given by trained healthcare professionals.

Illicit synthetic opioids are different. They are often:

• Made in unregulated labs
• Mixed into other drugs, such as heroin, cocaine, methamphetamine, or benzodiazepines (sedatives sometimes used to treat anxiety or sleep problems), sometimes without a person’s knowledge
• Pressed into counterfeit pills that look like prescription medications, including oxycodone or alprazolam (Xanax)

Global drug monitoring agencies, including the United Nations Office on Drugs and Crime, have warned that fentanyl and similar substances are increasingly found in drugs sold as heroin, methamphetamine, or counterfeit pain pills.

That means a person may take a substance believing it’s one thing when it actually contains a powerful synthetic opioid. Because these drugs can be much stronger than expected, even a small amount may raise the risk of overdose. This loss of control over what’s being used is a major reason overdose risk has increased in recent years.

Why Synthetic Opioids Are So Dangerous

Synthetic opioids carry several overlapping risks that make them especially dangerous.

Extreme Potency

Fentanyl and related compounds bind strongly to opioid receptors. Small changes in dose can lead to big changes in effect. This increases the risk of overdose, especially when a person doesn’t know what they’re taking.

Some newer synthetic opioids, including nitazenes, can be extremely potent. Research on these drugs is still limited, but some reports suggest certain nitazenes may be even stronger than fentanyl.

Unpredictable Dosing

Illegal drugs are not evenly mixed, especially those made or sold outside regulated systems. One pill or powder sample may contain a small amount of fentanyl. Another may contain a potentially deadly amount.

This mix of unpredictable dosing and high potency helps explain why overdose deaths have increased in many countries. Illegally made fentanyl is often mixed into heroin and other drugs without a person’s knowledge. Even people with high tolerance may not be protected if the dose varies unexpectedly.

Hidden Exposure

People may not know they are taking a synthetic opioid at all. Fentanyl has been found in counterfeit pills made to look like oxycodone, alprazolam, or other prescription opioids.

This creates risk even for people who are not intentionally seeking synthetic opioids. It also increases risk after periods of reduced use, when tolerance is lower.

Rapid Effect on Breathing

Opioids, including synthetic opioids, can slow or stop breathing. This is called respiratory depression. These drugs affect the part of the brain that controls breathing, weakening the body’s normal drive to breathe.

Synthetic opioids may affect breathing very quickly. That’s why an overdose can become life-threatening within minutes, especially with highly potent synthetic opioids. In some cases, people may not have time to realize what’s happening.

Overdose Risk and Multiple Substance Use

One of the most important facts about overdose deaths is that they often involve more than one substance.

Opioids slow activity in the central nervous system (the system that controls functions like breathing, heart rate, and alertness). Other substances can make these effects much stronger.

High-risk combinations include:

• Opioids and benzodiazepines, such as alprazolam or diazepam (Valium)
• Opioids and alcohol
• Opioids and other sedatives (drugs that slow brain activity)

These substances all depress the central nervous system. Together, they can sharply increase the risk of breathing failure.

It’s important to be clear that not all sedating drugs are opioids. Benzodiazepines and alcohol are not opioids, but they can still become dangerous when mixed with opioids.

Many overdose deaths involve more than one substance, which can make the risk much higher than using a single drug alone.

Warning Signs of an Opioid Overdose

An opioid overdose is a medical emergency. Synthetic opioids can make symptoms appear quickly.

Key warning signs include:

• Slow, shallow, or stopped breathing
• Slow heartbeat
• Unresponsiveness or being unable to wake up
• Blue, gray, or pale lips or fingertips, depending on a person’s skin tone
• Very small “pinpoint” pupils

Even if a person is still breathing, very slow breathing is a serious warning sign.

Other signs may include choking sounds, vomiting, or a body that feels limp. When in doubt, it’s safer to treat the situation as a possible overdose and seek emergency help.

What To Do in an Overdose

If you think someone is overdosing:

• Call emergency services immediately.
• Give naloxone if it’s available.
• Roll the person onto their side to help keep the airway open and reduce the risk of choking.
• Try to keep the person awake and breathing.
• Stay with the person until emergency help arrives.

Naloxone is a medication that can reverse opioid effects by blocking opioid receptors in the brain and restoring breathing. Naloxone usually works quickly — within two to three minutes — but more than one dose may be needed, especially with strong synthetic opioids like fentanyl.

Even if the person wakes up, they will still need emergency medical care. Naloxone works for a limited time — about 30 to 90 minutes — while many opioids can stay in the body longer. Because of this, breathing can slow or stop again after naloxone wears off.

Why Awareness Matters With Opioid Use Disorder

For people living with opioid use disorder, synthetic opioids have changed the risk landscape. Drugs may be more potent, less predictable, and more likely to contain hidden substances, including illegally made fentanyl.

But learning more about these drugs can help reduce harm.

Understanding what synthetic opioids are and how they behave in the body can help people make safer decisions, recognize the signs of an overdose more quickly, and seek help sooner.

Treatment options also exist and can support recovery. These may include:

• Medications for opioid use disorder (MOUD)
• Counseling
• Behavioral therapy (treatment that helps people change patterns related to substance use)
• Peer support

Join the Conversation

On MyOpioidRecoveryTeam, people share their experiences with opioid use disorder, get advice, and find support from others who understand.

Do you have questions about synthetic opioids? Let others know in the comments below.