Fentanyl Addiction: Signs, Symptoms, and Side Effects

Fentanyl is one of the most powerful and deadly opioids in use today, no matter if you use the drug intentionally or unknowingly. Small amounts of fentanyl can lead to serious side effects, addiction, or overdose. 

If you’re wondering whether you or someone you love is showing signs of fentanyl use, this page breaks down the most common physical, mental, and behavioral symptoms of fentanyl addiction. Knowing what to look for could save a life and help you take the first step toward recovery.

Don’t Wait: Call for Fentanyl Addiction in California

What Is Fentanyl?

Fentanyl is a fully synthetic opioid. Laboratories manufacture the substance entirely from synthetic compounds, unlike morphine or codeine, which come from the opium poppy.1 

Scientists designed fentanyl’s structure to bind more tightly and efficiently to opioid receptors in the brain than natural opiates, producing: 

  • A more intense high
  • Faster onset
  • Stronger respiratory depression

This level of potency is impossible to achieve with natural compounds. Doctors can effectively use fentanyl to treat chronic pain and support recovery after surgery, but misusing the drug makes it extremely hazardous.

image of Fentanyl Citrate anesthetic solution vial isolated on white.

What Are the Side Effects of Fentanyl?

Whether you are using fentanyl under the supervision of a medical professional or are experiencing fentanyl addiction, the substance can cause side effects. 

Common side effects of fentanyl happen to 1 in 100 people.2 Some of these symptoms include:

  • Constipation
  • Nausea or vomiting
  • Stomach pain
  • Sleepiness
  • Dizziness or vertigo
  • Confusion
  • Headaches
  • Itching or skin rashes

Serious Side Effects

Fewer than 1 in 100 people experience serious side effects from fentanyl. However, these effects require immediate attention. If you are experiencing any of the following symptoms, call 911 (and remove the fentanyl patch if you are wearing one):

  • Stiff muscle
  • Dizzy, tired, and low energy
  • Short, shallow breathing
  • Seizure
  • Swollen lips, mouth, throat, or tongue 
  • Skin, tongue, or lips turn blue
  • Sudden confusion

Long-Term Side Effects

Misusing fentanyl can permanently damage a person’s physical health, mental well-being, and quality of life. People who use the drug over an extended period of time may experience these side effects:

  • Emotional numbness or apathy (anhedonia) 
  • Cognitive decline
  • Muscle weakness
  • Immune system suppression
  • Financial instability
  • Chronic constipation
  • Increased risk of overdose

Signs of Fentanyl Addiction

Since fentanyl is a highly potent opioid, its side effects can appear quickly. Some of these symptoms of fentanyl use can be severe and require medical attention or treatment right away. Signs of fentanyl use can be mental, physical, and behavioral:

Mental Signs

  • Extreme euphoria followed by deep sedation or confusion
  • Mood swings or emotional numbness
  • Anxiety, especially when the drug wears off
  • Disorientation, appearing dazed or “out of it”
  • Hallucinations or altered perception (in higher doses or mixed substances)
  • Paranoia or defensiveness about whereabouts and activities

Physical Signs

  • Pinpoint pupils
  • Drowsiness
  • Slowed or labored breathing
  • Flushed skin, itching, or rashes
  • Nausea, vomiting, or constipation
  • Poor coordination or slurred speech
  • Sudden weight loss or signs of malnutrition
  • Unusual sweating, chills, or tremors
  • Frequent nosebleeds (if snorted) or track marks (if injected)

Behavioral Signs

  • Doctor shopping or frequent requests for pain medication
  • Missing money, valuables, or prescription drugs
  • Secretive behavior or isolation from family/friends
  • Neglecting responsibilities at work, school, or home
  • Erratic sleep patterns
  • Increased risk-taking or dangerous behavior
  • Lying about activities, injuries, or physical state

Fentanyl acts fast, which means that these signs of use can quickly escalate into life-threatening situations. Call 911 immediately, and administer naloxone (Narcan) if someone is showing these signs:

  • Blue lips or fingertips
  • Unresponsiveness
  • Gurgling or gasping for breath

These are signs of a fentanyl overdose. Acting quickly can help save a life.

Do I Have a Fentanyl Addiction?

If you’re wondering whether you’re addicted to fentanyl, trust your gut. If something feels wrong, you’re probably right. The fastest and most accurate way to know is to call our admissions team for a free, confidential assessment.

The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) outlines clear criteria for diagnosing opioid use disorder (OUD).3 If you’ve experienced two or more of the following symptoms in the past 12 months, it may be time to seek help:

  1. Persistent use despite physical or psychological harm.
  2. Ongoing use despite relationship problems.
  3. Withdrawal from social or recreational life.
  4. Neglect of responsibilities.
  5. Significant time spent on opioid-related activities.
  6. Taking more than intended.
  7. Strong desire or craving to use.
  8. Unsuccessful attempts to cut back.
  9. Development of tolerance.
  10. Use continues despite known risks.
  11. Withdrawal symptoms or avoidance of withdrawal.

Opioid use disorder is treatable. With medical detox and evidence-based therapy, recovery is possible.

Signs of Fentanyl Overdose

A fentanyl overdose can happen quickly and without warning. Because the drug is highly potent, recognizing the signs of a fentanyl overdose can be what saves the life of a loved one.

Signs of fentanyl overdose include:4

  • Small, constricted (“pinpoint”) pupils
  • Loss of consciousness or falling asleep and not waking up
  • Unresponsive to shouting, rubbing knuckles on the sternum, or pinching
  • Breathing is very slow, shallow, erratic, or has stopped
  • Choking or gurgling sounds
  • The body is limp
  • The face is pale and clammy
  • Fingernails and lips turn blue
  • Slow, erratic, or absent pulse (heartbeat)
  • Vomiting

If you believe someone is experiencing an opioid overdose, California health authorities recommend: 

  • Call 911 immediately
  • Administer naloxone (Narcan) if available
  • Try to keep the person awake and breathing
  • Turn the person on their side to prevent choking
  • Stay with the person until emergency services arrive

California Poison Control System can assist in the management of a suspected fentanyl overdose. You can reach them at 1-800-222-1222.5 Don’t be afraid to call first responders to save someone’s life. California’s Good Samaritan Law protects people from civil liability when they provide emergency medical or non-medical care at the scene of an emergency.6

FAQs About Fentanyl Side Effects

Fentanyl exposure, typically through skin contact or inhalation of powder, can cause sudden drowsiness, dizziness, confusion, slow or shallow breathing, pinpoint pupils, clammy skin, nausea, and blue lips or fingertips. Muscles may become rigid or limp.

If you suspect exposure, move to fresh air, administer naloxone if available, and call 911 immediately.

The effects of fentanyl begin within seconds to minutes and typically last 1 to 2 hours. However, respiratory depression and sedation may persist longer.

The most common side effects of fentanyl are constipation, nausea, drowsiness, dizziness, confusion, headache, and itching or rash. While often mild, some symptoms, especially sedation and respiratory slowing, can become dangerous at higher doses or when mixed with other substances.

Your body can retain fentanyl for 24 to 72 hours, depending on the dose, frequency of use, metabolism, and how you take it. Drug tests can detect it in:

  • Urine: up to 3 days
  • Blood: up to 48 hours
  • Saliva: up to 2 days
  • Hair: up to 90 days

Detection windows vary, especially with long-term use.

Fentanyl withdrawal symptoms typically begin 8 to 24 hours after the last dose, depending on the type used (short-acting vs. patch), the dose, and how long it was used. Symptoms usually peak within 2 to 4 days, sometimes lasting up to 10 days or longer.

Next Step: Get Help for Fentanyl Addiction in California

Fentanyl addiction can escalate to a life-threatening situation quickly, but recovery is possible. If you or someone you love is showing signs of fentanyl misuse, Tree House Recovery can help. We specialize in helping individuals heal from the effects of fentanyl addiction and rebuild their lives with confidence, purpose, and long-term support.


Call (855) 202-2138 now for a free, confidential consultation with our admissions team. We’re available 24/7 to help you take the first step toward recovery. You can also learn more about our fentanyl addiction treatment programs in California.

  1. National Institute on Drug Abuse. Fentanyl. National Institute on Drug Abuse. Published December 21, 2021. https://nida.nih.gov/research-topics/fentanyl
  2. NHS. Side Effects of Fentanyl. nhs.uk. Published May 25, 2023. https://www.nhs.uk/medicines/fentanyl/side-effects-of-fentanyl/
  3. Dydyk AM, Jain NK, Gupta M. Opioid Use Disorder. PubMed. Published January 17, 2024. https://www.ncbi.nlm.nih.gov/books/NBK553166/
  4. Cultivating Health. Fentanyl Facts, Overdose Signs to Look For, and How You Can Help Save a Life. cultivating-health. Published January 11, 2023. https://health.ucdavis.edu/blog/cultivating-health/fentanyl-overdose-facts-signs-and-how-you-can-help-save-a-life/2023/01
  5. Substance and Addiction Prevention Branch. Responding to a Fentanyl Overdose: What California First Responders Need to Know. www.cdph.ca.gov. Published January 2022. https://www.cdph.ca.gov/Programs/CCDPHP/sapb/
  6. Understanding California’s 911 Overdose Good Samaritan Law. Sacramento County Department of Health Services, 2022.
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