When you overdose, your body is overwhelmed by more of a substance than it can safely process, and vital functions start to fail. What specifically goes wrong depends on the type of substance involved, but the most dangerous outcomes center on three things: your breathing slows or stops, your heart rhythm becomes unstable, or your organs begin to shut down. An overdose can be fatal within minutes in some cases, or it can unfold over hours or even days with substances that damage organs gradually.
How Opioid Overdoses Affect Breathing
Opioids, including prescription painkillers, heroin, and fentanyl, kill by shutting down the brain’s drive to breathe. These drugs bind to receptors concentrated in breathing centers scattered throughout the brainstem. When too many of these receptors are activated at once, the neurons responsible for maintaining your breathing rhythm become suppressed. Your breaths grow shallow and infrequent, and in severe cases, they stop entirely.
Once breathing fails, oxygen levels in the blood plummet. Within four to six minutes of oxygen deprivation, brain cells begin to die. The lips and fingertips turn blue or gray. The person may make gurgling or snoring sounds as the airway partially collapses. They typically cannot be woken up. Fentanyl makes this scenario especially dangerous because as little as 2 milligrams, roughly the size of a few grains of salt, can be lethal. Its effects also hit faster than heroin or morphine, shrinking the window for intervention.
What Happens With Depressant Combinations
Alcohol, benzodiazepines (like Xanax or Valium), and opioids are all central nervous system depressants, and combining them is one of the most common paths to a fatal overdose. Most deaths from benzodiazepine overdose specifically involve a combination with another depressant, particularly alcohol or opioids. Each substance amplifies the sedating effects of the others, pushing the body into deeper suppression than any single drug would cause alone.
Beyond slowed breathing, these combinations impair your reflexes in a way that creates a secondary danger: aspiration. When someone is deeply sedated, the gag reflex that normally prevents vomit from entering the lungs stops working. If the person vomits while unconscious and lying on their back, they can choke or develop a severe lung infection. This is why body positioning matters so much in the minutes before help arrives.
Stimulant Overdoses Work Differently
Overdoses on stimulants like cocaine or methamphetamine don’t suppress breathing. Instead, they push the cardiovascular system into overdrive. Heart rate spikes, blood pressure climbs dangerously high, and the heart can slip into an irregular rhythm that prevents it from pumping blood effectively. Seizures are common because the brain is being flooded with far more stimulation than it can handle.
Body temperature is another critical factor. Stimulant overdoses can cause hyperthermia, where core temperature rises to levels that begin damaging muscles, kidneys, and the brain. Unlike opioid overdoses, there is no reversal drug for stimulant toxicity. Treatment is entirely focused on managing each symptom as it appears: cooling the body, controlling seizures, and stabilizing the heart.
Acetaminophen and Slow-Building Organ Damage
Not every overdose is dramatic or immediately obvious. Acetaminophen (Tylenol) overdose is one of the leading causes of acute liver failure, and it can take 24 to 72 hours before serious symptoms appear. In the first several hours, a person may feel only mild nausea or nothing at all. Meanwhile, a toxic byproduct of the drug is accumulating in the liver, destroying cells.
For adults, the threshold dose that triggers liver damage is roughly 10 to 15 grams, which is the equivalent of 20 to 30 extra-strength tablets taken within a short period. For children, the threshold is around 150 milligrams per kilogram of body weight. This makes acetaminophen overdose particularly deceptive. People sometimes take too much over the course of a day without realizing it, especially if they’re combining multiple products that contain it. By the time jaundice, abdominal pain, and confusion set in, the liver may already be severely damaged. An antidote exists, but it works best when given within the first 8 to 10 hours.
Lasting Damage From Surviving an Overdose
Surviving an overdose does not always mean recovering fully. The most significant long-term risk comes from the period when the brain was deprived of oxygen, a condition called hypoxic brain injury. A systematic review of studies on people who survived opioid overdoses found a wide range of lasting impairments. Cognitive symptoms were the most commonly reported, appearing in 58 subjects across the reviewed studies. These included confusion, memory loss (both short-term and long-term), difficulty paying attention, slowed thinking, and impaired decision-making.
Physical consequences were also common. Involuntary movements like tremors and dyskinesia appeared in 40 subjects. Others experienced problems with walking and balance, muscle rigidity, limb weakness, or partial paralysis. Some people developed psychiatric and emotional changes, including apathy, social withdrawal, aggression, and odd behavior. The severity of these outcomes depends largely on how long the brain went without adequate oxygen and how quickly medical care was delivered.
What to Do If Someone Is Overdosing
Call emergency services immediately. For a suspected opioid overdose, naloxone nasal spray (Narcan) can reverse the effects. The standard dose is one spray delivering 4 milligrams into one nostril. If the person doesn’t wake up or start breathing normally within 2 to 3 minutes, give a second dose using a new spray in the other nostril. You can keep giving doses every 2 to 3 minutes until help arrives. One important detail: some opioids, especially fentanyl, outlast naloxone in the body. A person who initially responds can slip back into respiratory depression as the naloxone wears off, so continuous monitoring is essential even after they wake up.
If the person is unconscious and breathing, place them in the recovery position to keep their airway clear. Kneel beside them, extend the arm nearest you straight out with the palm facing up, then fold their other arm so the back of that hand rests against their nearest cheek. Bend their far knee to a right angle, then roll them toward you by pulling that bent knee. Their head should rest on the folded hand, and the bent leg keeps them stable on their side. Gently tilt the head back and lift the chin to open the airway. This position prevents choking if they vomit.
For overdoses involving stimulants, acetaminophen, or unknown substances, there is no at-home reversal. The priority is calling for help, keeping the person safe from injury (especially during seizures), and noting what substance was taken and approximately when. If you can bring the pill bottle or packaging to the emergency room, that information significantly speeds up treatment decisions.