A drug overdose happens when a substance overwhelms the body’s ability to maintain its basic functions, most critically breathing and heart rhythm. In 2024, 79,384 people died from drug overdoses in the United States, with synthetic opioids like fentanyl involved in roughly 47,700 of those deaths. The way an overdose unfolds depends on the type of drug involved, but in nearly every case, it comes down to one of two pathways: the body slows down too much or speeds up too far.
Opioid Overdoses Shut Down Breathing
Opioids, including heroin, fentanyl, and prescription painkillers, kill by suppressing the drive to breathe. Deep in the brainstem, a small cluster of roughly 800 to 1,000 neurons on each side generates the rhythm that triggers every breath you take. Opioids bind to receptors on these neurons and quiet them. They also act on nearby brain structures that control how fast and how regularly you breathe. The result is breathing that becomes slower, shallower, and increasingly irregular until it stops altogether.
Once breathing fails, oxygen levels in the blood plummet. The brain, heart, and other organs begin to starve. Skin turns bluish, especially around the lips and fingertips. The person may make gurgling or choking sounds, sometimes mistaken for snoring. Pupils shrink to pinpoints. Without intervention, the heart eventually stops.
What makes opioid overdose especially dangerous is how narrow the margin can be. Fentanyl is about 50 times stronger than heroin and 100 times stronger than morphine. As little as 2 milligrams of fentanyl, roughly 10 to 15 grains of table salt, is considered a lethal dose. That tiny margin means even a small miscalculation in dose, or an unexpected encounter with fentanyl mixed into another drug, can be fatal.
Stimulant Overdoses Push the Body Too Hard
Cocaine, methamphetamine, and other stimulants cause overdose through the opposite mechanism. They flood the brain with signaling chemicals that activate the body’s fight-or-flight system. Heart rate and blood pressure spike. Body temperature climbs. The heart can be driven into a dangerous irregular rhythm or simply pushed past its limits.
As stimulant toxicity progresses, the cascade gets worse: seizures, muscle breakdown that damages the kidneys, bleeding in the brain, and body temperatures high enough to cause organ failure. Unlike opioid overdoses, which tend to look like someone falling into a deep, quiet sleep, stimulant overdoses are often chaotic. The person may be agitated, confused, hallucinating, drenched in sweat, with wide pupils and a pounding pulse.
Mixing Substances Multiplies the Risk
Many overdose deaths involve more than one drug, and combining substances is one of the most common ways people overdose without intending to. The combination of opioids with alcohol or sedatives like benzodiazepines (Xanax, Valium) is particularly lethal because each substance suppresses breathing through a different mechanism. Opioids quiet the brainstem’s breathing centers directly. Alcohol and benzodiazepines amplify the brain’s main “slow down” signal while also dampening its main “speed up” signal. Together, these effects stack, producing far greater respiratory depression than either substance would alone.
Someone who can tolerate a certain dose of an opioid on its own may stop breathing at that same dose after a couple of drinks. The subjective high may feel stronger too, since benzodiazepines are reported to enhance the euphoric effects of opioids, which can lead people to misjudge how much danger they’re in.
Accidental Overdose Is More Common Than Intentional
Most overdoses are not deliberate. They happen because of a few common scenarios. One is contamination of the drug supply. Fentanyl is now routinely found mixed into cocaine, methamphetamine, counterfeit pills, and heroin. You can’t see, taste, or smell it. A person who uses cocaine and has no opioid tolerance can be killed by a trace amount of fentanyl they didn’t know was there.
Another common scenario is a change in tolerance. Someone who stops using opioids for a period, whether through a stint in jail, a hospital stay, or an attempt at quitting, loses their built-up tolerance quickly. If they return to the same dose they used before, it can overwhelm a body that has reset to baseline.
Using alone is another major factor. When no one else is present, there’s no one to call for help or administer naloxone if breathing stops.
What an Overdose Looks Like
Recognizing an overdose depends on what type of drug is involved. For opioids, the signs are slow or absent breathing, unresponsiveness, pinpoint pupils, pale or bluish skin, and limpness. The person may look like they’re in a very deep sleep and can’t be woken up.
For stimulants, look for a racing pulse, chest pain, extreme agitation or confusion, seizures, very high body temperature, and profuse sweating with dilated pupils. The person may be conscious but clearly in distress.
For combinations of depressants (opioids plus alcohol, for example), the signs resemble an opioid overdose: slowed breathing, unresponsiveness, and loss of consciousness.
Naloxone Can Reverse Opioid Overdoses
Naloxone (sold as Narcan) is a medication that binds to the same receptors as opioids and displaces them, restoring breathing. When given intravenously, it can work within one minute. Nasal spray versions take slightly longer but are designed for bystanders to use without medical training.
One important complication: fentanyl overdoses often require higher doses of naloxone than overdoses involving heroin or prescription painkillers. Emergency providers may need to administer 10 milligrams or more, compared to smaller doses that once sufficed. Naloxone can also be given every 2 to 5 minutes if the first dose doesn’t restore adequate breathing. Because fentanyl can outlast a single dose of naloxone, someone who initially responds may slip back into respiratory depression and need additional treatment.
Naloxone only works on opioids. It has no effect on stimulant overdoses, alcohol poisoning, or benzodiazepine toxicity.
Surviving an Overdose Can Cause Lasting Damage
An overdose that doesn’t kill can still leave permanent harm. The core problem is oxygen deprivation. When breathing stops or slows dramatically, the brain takes the worst hit. The hippocampus, the brain region responsible for forming new memories, is especially vulnerable to low oxygen levels.
Non-fatal opioid overdoses are associated with cognitive deficits, memory problems, depression, fluid in the lungs, pneumonia, seizures, and irregular heart rhythms. Researchers using brain imaging have found that people with a history of overdose have measurably smaller hippocampal volume than those who haven’t overdosed. About 20 documented cases in the U.S. and France describe a specific amnestic syndrome, where opioid users developed severe memory impairment traced to acute hippocampal injury after an overdose.
Perhaps the most sobering statistic: surviving one overdose significantly increases the risk of a subsequent fatal overdose.
Fentanyl Test Strips and Harm Reduction
Because fentanyl contamination in the drug supply is invisible, fentanyl test strips have become an important tool. These small paper strips can detect fentanyl in cocaine, methamphetamine, heroin, pills, powders, and injectables. They don’t eliminate risk, but they give people information they otherwise wouldn’t have. The CDC promotes them as a way for people who use drugs and their communities to take steps to reduce overdose risk. Test strips are inexpensive and increasingly available through public health programs and pharmacies in many states.