Medically Reviewed

Fentanyl Side Effects: Mental & Physical Effects of Fentanyl Use

3 min read · 2 sections

The Center for Disease Control (CDC) reports that 67% of all drug overdoses between January 2021 and January 2022 involved fentanyl.1 It’s no wonder that fentanyl is such a prominent contributor to overdoses; ingesting only 2 milligrams of this substance can cause death.1

Fentanyl carries many potential adverse side effects that can be harmful to a person’s health when consumed alone or in combination with other drugs.

Because of fentanyl’s profound level of potency and growing prominence in society, it’s critical to understand where fentanyl comes from, its risks and adverse effects, and its role in overdoses in the United States. Through gaining more context about fentanyl, you can help keep yourself and your loved ones safe from this substance and its negative implications.

What is Fentanyl?

Fentanyl is a fully synthetic opioid 50 to 100 times more potent than morphine.2 Prescription fentanyl may be prescribed to treat relatively severe pain. In its various formulations, prescription fentanyl may be administered as an injectable solution, transdermal patch, or lozenge.2

Synthetic opioids, including fentanyl, are the most cited types of drugs involved in overdose deaths in the United States.2 Though cases of non-medical misuse of pharmaceutical fentanyl exist, much of the opioid overdose crisis in recent years has involved illicitly manufactured fentanyl. Such illicit forms of this drug are sometimes sold in powder form or pressed into pills that often resemble other prescription medications.2

People who purchase illicit drugs, such as heroin, meth, or cocaine, may accidentally purchase a substance laced with fentanyl without knowing it. For someone who has no tolerance to opioids whatsoever, this could be especially dangerous. Some drug dealers also advertise and sell fentanyl under the guise of extremely strong heroin, posing profound danger to unknowing purchasers.3

Anyone who uses fentanyl is subject to experiencing a range of adverse (and potentially deadly) effects. Fentanyl use of any kind is potentially even more risky for people who suffer from obstructive airway diseases (such as asthma), suffer from liver failure, are hypersensitive to or allergic to certain fillers in fentanyl, or whose tolerance to opioids is low.3

Opioids like fentanyl can lead to dangerously slowed breathing.4 If left untreated, respiratory depression can progress to full respiratory arrest, or the complete shut-down of respiratory function. As such, fentanyl misuse can increase the risk of hypoxia (impaired delivery of oxygen to vital tissues), hypoxic brain injury, and even death.4

Polysubstance use can pose additional risks to people who consume fentanyl. The presence of other drugs, such as alcohol and opioids, can exacerbate fentanyl’s potential negative effects and create complex and dangerous medical scenarios.3

As is the case with all opioids, one major potential adverse effect of fentanyl misuse is that it increases the likelihood of someone developing a substance use disorder (SUD), or addiction to fentanyl.5 Such a substance use disorder, or an opioid use disorder involving fentanyl, is characterized by someone’s inability to stop using fentanyl, despite negative life consequences—such as those related to work, school, and social/personal affairs.3

In some cases, fentanyl may be mixed in with drugs that someone uses a needle to inject. Drug use by needle injection may be associated with dangers aside from the adverse pharmacological effects of the drug itself.6 For example, If someone injects fentanyl or another drug mixed with fentanyl with a nonsterile needle, their risk of contracting HIV may increase substantially.6 People that inject fentanyl through nonsterile needles also risk the potential of developing endocarditis (infection of the heart valves), tetanus, abscesses, and more.

Fentanyl Overdose

If you believe that you or someone you love is overdosing on fentanyl or another drug, this is a medical emergency. Call 911 immediately or visit your nearest emergency medical center.

Overdose is perhaps the most dangerous and potentially impactful potential adverse effect of fentanyl consumption. Fentanyl’s high level of potency greatly increases its risk for overdose and other adverse reactions.3

Synthetic opioids, which include fentanyl, are often involved in overdose deaths.2 Fentanyl’s role in overdoses today is partially due to many illegal drug distributors intentionally mixing fentanyl with other substances, like methamphetamine, heroin, and cocaine, to drive down their costs of production (and therefore increase their profits).2 For people who have built no tolerance to opioids – and even for those who have – consuming fentanyl unknowingly with other drugs can be extremely dangerous.3

It’s often easier and more cost-effective for drug dealers to mass-produce illicit opioids by “cutting” their products with a small amount of fentanyl.2 Therefore, people who purchase these drugs may end up consuming fentanyl without knowing it, which can result in someone taking too much fentanyl and overdosing.

Unfortunately, it can be practically impossible to distinguish with the naked eye whether illicitly-manufactured drugs, such as heroin, MDMA, cocaine, and methamphetamine, contain trace amounts of fentanyl.6 For example, a legally manufactured oxycodone pill and a counterfeit one laced with fentanyl may be practically indistinguishable. Both might be round and blue and may contain the same markings.6 It can reportedly take just two milligrams ) to fatally overdose on fentanyl.6

However, there are three symptoms often recognized as the classic “opioid overdose triad” when it comes to toxic doses of opioids, including fentanyl. These include:7

  • Small pupils.
  • Respiratory depression (slow, ineffective breathing).
  • Loss or decreased level of consciousness.

Is Fentanyl Overdose Reversible?

Yes, fentanyl overdose is reversible with a drug called naloxone and prompt medical attention. Reversal of fentanyl overdose is possible if appropriate action is taken in a timely manner.8 Contact 911 immediately once overdose symptoms present themselves.8 Good Samaritan laws will protect you from criminal action if you report an overdose.8 So, don’t let the fear of criminal action prevent you from saving someone’s life.

Naloxone is a life-saving drug that rapidly reverses an opioid overdose.9 Naloxone is an opioid antagonist, meaning it binds to the opioid receptors in the body and blocks the effects of any circulating opioid agonist drug.9 It is only effective in countering the effects of opioids and may not work to fully reverse overdoses involving other substances.9

Multiple rounds of naloxone may be needed to reverse an overdose on fentanyl, because fentanyl is such a strong opioid.9 Fentanyl’s potency should be taken into consideration when administering naloxone.9 There are ongoing efforts to formulating a stronger formulation of naloxone to counter the effects of extremely potent opioids, including fentanyl.9

Naloxone can be given as a spray or injection.9 The effectiveness of this substance is so profound that overdose kits containing the substance are being given to police officers, non-medical first responders, and even family or friends of a person that has a history of opioid use.9 Community based programs or local health departments can also provide training on proper naloxone use and may even provide the substance free of charge to the public, depending on a state’s laws.9

Again, please be sure to call emergency services as soon as possible if it’s believed that someone is overdosing on opioids.9 Medical professionals may need as much time as possible to administer naloxone if more than one dose is needed.9


  1. United States Drug Enforcement Administration. (n.d.). Fentanyl awareness.
  2. National Institute on Drug Abuse. (2021, June 1). Fentanyl DrugFacts.
  3. Ramos-Matos CF, Bistas KG, Lopez-Ojeda W. (2022). Fentanyl.
  4. Kiyatkin, E. A. (2019, June). Respiratory depression and brain hypoxia induced by opioid drugs: Morphine, oxycodone, heroin, and fentanyl. Neuropharmacology, 151, 219–226.
  5. National Institute on Drug Abuse. (2019, January 17). Treatment approaches for drug addiction DrugFacts.
  6. U.S. Department of Justice. (2021, December 8). Acting U.S. Attorney warns of increasing danger of counterfeit prescription opioids containing fentanyl.
  7. Schiller, E. Y., Goyal, A., & Mechanic, O. J. (2022, May 9). Opioid overdose. StatPearls.
  8. National Institute on Drug Abuse. (2022, May 12). What is fentanyl?
  9. National Institute on Drug Abuse. (2022, January 11). Naloxone DrugFacts.
  10. National Institute on Drug Abuse. (2019, January 17). Treatment approaches for drug addiction DrugFacts.
  11. National Institute on Drug Abuse. (2016, November 1). Effective treatments for opioid addiction.
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