Fentanyl Side Effects

Naomi Carr
Morgan Blair
Written by Naomi Carr on 19 October 2023
Medically reviewed by Morgan Blair on 21 December 2023

Fentanyl is a very strong and fast-acting synthetic opioid, used as a prescription painkiller and as an illicit street drug. Fentanyl causes several side effects, including drowsiness, fatigue, euphoria, and nausea, and can cause severe effects, particularly when misused.

Fentanyl Side Effects

What is fentanyl?

Fentanyl is a synthetic opioid medication, also known as a narcotic. It is approved for medicinal use, although it is also produced and used illicitly. Through a prescription, fentanyl is available in various forms, including a buccal tablet, transdermal patch, and lozenge.

What is fentanyl used for?

Fentanyl is an analgesic, or pain relief, medication. It is prescribed to opioid-tolerant individuals with cancer who require consistent pain relief or to treat severe pain following surgery. Fentanyl is fast-acting and extremely potent, so it is an effective pain medication. It is 50 times stronger than heroin and 100 times stronger than morphine.

Why do people abuse fentanyl?

Fentanyl causes euphoric, sedating, and relaxing effects, which contribute to its common misuse and abuse. It is also highly addictive, so it can be very difficult to stop using this substance after a dependence or addiction has formed. It is common for people to begin abusing fentanyl following an opioid prescription.

For example, someone may have been prescribed fentanyl as a pain relief. If they continue this prescription for some time, they are likely to develop a tolerance. They might then begin to misuse their medication, taking larger and more frequent doses than they are prescribed, in order to achieve the desired effect. 

Similarly, someone prescribed fentanyl or another opioid medication might form a dependence or addiction to their medication. If this prescription is suddenly stopped, they are likely to experience withdrawal symptoms and cravings. This could cause them to seek and buy fentanyl illegally to satisfy their cravings. 

Common side effects of fentanyl

When starting fentanyl treatment, it may be common to experience some side effects. Often, these will reduce on their own without the need for treatment. If any of these side effects persist or become concerning, it is advised to consult with the prescribing doctor.

Common side effects include:

  • Nausea
  • Vomiting
  • Constipation
  • Drowsiness
  • Fatigue and weakness
  • Headache
  • Low mood
  • Anxiety
  • Insomnia
  • Increased or unusual dreams
  • Shortness of breath
  • Shaking
  • Weight loss
  • Swelling of the arms or legs
  • Redness in the face or neck
  • Difficulty urinating or urinary incontinence

Severe side effects of fentanyl

Fentanyl use can sometimes cause severe side effects. If any of the following occur, it is recommended to seek professional advice and treatment:

  • Irregular heartbeat
  • Chest pain and tightness
  • Trouble breathing
  • Shallow and slow breaths
  • Extreme weakness and fatigue
  • Fainting
  • Seizures
  • Severe or persistent dizziness
  • Persistent vomiting or diarrhea
  • Sexual dysfunction
  • Irregular or stopped menstruation
  • Rash or hives
  • Fever
  • Hallucinations
  • Disordered thoughts or confusion
  • Overdose 

Fentanyl can cause the development of tolerance, dependence, and addiction, whether used as prescribed or illicitly. 

Tolerance occurs when the drug has been used for a prolonged period and no longer has the same effect, requiring higher doses to achieve the same level of pain relief or ‘high’.

Dependence occurs when the drug has been used for an extended period and the body becomes reliant upon its effects. This causes withdrawal symptoms when the substance is no longer used, contributing to its continued use. Withdrawal symptoms can include extreme changes in sleep, appetite, and mood, vomiting and diarrhea, and drug cravings.

Addiction is a psychological condition that often occurs alongside physical dependence. It causes compulsive behaviors relating to drug seeking and use that have harmful effects on the individual’s life and functioning.

When fentanyl is misused or used illicitly, the risk of dependence and addiction increases. Fentanyl abuse can also result in an increased risk of severe side effects, such as respiratory problems, loss of consciousness, and extreme weakness, along with a high risk of overdose.

Fentanyl overdose

Because of its potency, fentanyl carries a high risk of overdose, particularly if it is misused or abused. More people in the US overdose on fentanyl or other synthetic opioids than any other drugs and the consequences can be fatal.

A contributing factor to the risk of overdose is the lacing or replacing of other drugs with fentanyl. Many drug dealers mix or replace drugs such as heroin, methamphetamine, and opioid pills with fentanyl because it is cheap to produce. 

The individual who then uses these drugs will often be unaware that they contain fentanyl. Even a small dose of fentanyl can be fatal for someone who has not used it before or developed a tolerance. As such, an unaware individual is likely to use an amount that causes an overdose, which may have fatal consequences.

Signs of a fentanyl overdose include:

  • Slowed or stopped breathing 
  • Extreme weakness and fatigue
  • Confusion
  • Pinpoint pupils
  • Blue lips or skin
  • Unconsciousness

Naloxone is a medication used to treat fentanyl overdose. It can stop the effects of the drug and reduce the impact of dangerous symptoms . People who are prescribed fentanyl might also be prescribed Naloxone to keep in their homes, in case of an emergency.

Getting treatment

Often, people who are prescribed fentanyl will be gradually tapered on and off their medication to prevent or reduce side effects and withdrawal symptoms. Medical professionals will monitor the individual during this process, to manage and treat any unpleasant or dangerous effects.

People who experience fentanyl abuse, dependence, or addiction may require treatment in a detox or rehab facility to safely reduce and stop their substance use. Professional support is provided in these facilities to support individuals in the reduction of addictive behaviors, withdrawal symptoms, and underlying emotional distress or trauma. 

Medications can be prescribed during fentanyl withdrawal, including buprenorphine, methadone, naltrexone, and lofexidine.

Individuals with an opioid use disorder, such as fentanyl addiction, often require ongoing support. This could include psychotherapy, 12-step programs such as Narcotics Anonymous, support groups, and holistic therapies. Ongoing support and treatment can help improve the recovery process, manage cravings and triggers, and maintain abstinence.

Was this page helpful?

Your feedback allows us to continually improve our information


  1. National Institute on Drug Abuse. (2021). Fentanyl Drug Facts. NIDA. Retrieved from
  2. Cephalon, Inc. (Revised 2011). Actiq (Fentanyl Citrate Oral Transmucosal Lozenge. FDA. Retrieved from
  3. Centers for Disease Control and Prevention. (Revised 2023). Fentanyl Facts. CDC. Retrieved from
  4. Drug Enforcement Administration. (2016). Fentanyl. DEA. Retrieved from
  5. Alcohol and Drug Foundation. (Updated 2023). Fentanyl. ADF. Retrieved from
  6. Dydyk, A.M., Jain, N.K., Gupta, M. (Updated 2023). Opioid Use Disorder. In: StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing. Retrieved from
  7. National Library of Medicine. (Revised 2023). Fentanyl. Medline Plus. Retrieved from
  8. Wilson, N., Kariisa, M., Seth, P., Smith IV, H., Davis, N.L. (2020). Drug and Opioid-Involved Overdose Deaths — United States, 2017–2018. Morbidity and Mortality Weekly Report (MMWR), 69(11), 290–297.

Activity History - Last updated: 21 December 2023, Published date:


Morgan Blair


Morgan is a mental health counselor who works alongside individuals of all backgrounds struggling with eating disorders. Morgan is freelance mental health and creative writer who regularly contributes to publications including, Psychology Today.

Activity History - Medically Reviewed on 15 October 2023 and last checked on 21 December 2023

Medically reviewed by
Morgan Blair


Morgan Blair


Ready to talk about treatment? Call us today. (855) 648-7288
Helpline Information
Phone numbers listed within our directory for individual providers will connect directly to that provider.
Any calls to numbers marked with (I) symbols will be routed through a trusted partner, more details can be found by visiting https://recovered.org/terms.
For any specific questions please email us at info@recovered.org.

More like this

Fentanyl Test Strips

5 minutes read


7 minutes read