Fentanyl IV Formula (Sublimaze®)

OncoLink Team
Last Modified: October 16, 2015

Pronounced: FEN-ta-nil
Classification: Opioid/Narcotic

About Fentanyl

Fentanyl and all opiate painkillers work by activating certain receptors in the brain that cause pain relief. Fentanyl increases one's pain threshold and inhibits certain nerve pathways that sense pain. Opiate painkillers are the strongest class of painkillers.

How to Take Fentanyl

There are many different formulations of fentanyl. This information is about fentanyl given intravenously (into a vein). When Fentanyl is injected, it is immediately absorbed into the body. It will begin working to relieve pain almost instantly, although it reaches its peak effect in 5 to 10 minutes. It will continue to work for 2 to 4 hours.

This medication can interact with alcohol and other medications that depress the central nervous system like barbituates (including phenobarbital), tranquilizers (including Haldol®, Librium® and Xanax®), other narcotics and general anesthetic. Make sure your provider is aware of all the medications, vitamins and supplements you are taking.

Possible Side Effects of Fentanyl

There are a number of things you can do to manage the side effects of fentanyl. Talk to your doctor or nurse about these recommendations. They can help you decide what will work best for you. These are some of the most common side effects:


Constipation is a very common side effect of pain medications, but one that can often be managed with an ounce of prevention. Be sure to drink 8-10 glasses of water a day. Warm or hot fluids can be helpful. Increase physical activity when possible. Attempt a bowel movement at the same time each day. It is helpful to eat plenty of fruits and vegetables. Some folks do find that 4 ounces of prune juice or 3-4 dried prunes/plums help promote bowel movements. However, high fiber foods (ex. bran flakes, high fiber cereals) and fiber supplements (such as Metamucil) can actually make constipation from pain medications worse and should be avoided.

Your doctor or nurse may recommend a bowel regimen, using stool softeners and/or laxatives, to prevent or treat constipation. Stool softeners (such as docusate sodium or Colace) work by bringing water into the stool, making it softer and easier to pass. A stimulant or laxative (such as Bisacodyl, Dulcolax, or Senakot) works by stimulating peristalsis, moving the stool through the bowel. Your provider may recommend Miralax (Polyethylene glycol 3350), which is an osmotic laxative. It works by causing water to be retained in the stool, softening the stool so it is easier to pass. These medications can be taken together. Untreated constipation can lead to a bowel blockage, so be sure to notify your healthcare team if you do not have a bowel movement for 3 or more days.

Sleepiness (Somnolence)

Feeling sleepy, drowsy or lightheaded may accompany the use of opioid painkillers. Some people just don't "feel like themselves" on these medications. Avoid driving or any other potentially dangerous tasks that require your concentration and a clear head until you feel normal again. Avoid alcohol or other sedatives while using these medications unless they are specifically prescribed by your doctor. Most people will begin to feel like themselves after a few days on the medications. If you continue to feel "out of it" after a couple of days, talk to your doctor about adjusting your dosages.

Slowed Breathing or Low Blood Pressure

You may experience low blood pressure or slowed breathing while taking an opioid painkiller. This usually only occurs when the dose of medication is too high or it is increased too quickly. This rarely happens to patients who have been taking opioid medications for a long time.

These side effects can also result from an overdose of opioids. If you suspect that you or someone you know has taken an overdose of opioids, call 911 immediately. If you feel extremely tired, lightheaded, dizzy, sweaty, nauseated, or short of breath, you need to see a doctor immediately. Sometimes patients who have taken too much opioid medication will be so sleepy that they can't be awakened or aroused. These side effects are emergency situations. If any of these symptoms occur, you should seek emergency medical attention.

Reproductive Concerns

Exposure of an unborn child to this medication could cause birth defects, so you should not become pregnant or father a child while on this medication. Effective birth control is necessary during treatment. Even if your menstrual cycle stops or you believe you are not producing sperm, you could still be fertile and conceive.  You should consult with your healthcare team before breastfeeding while receiving this medication.

Concerns About Addiction, Tolerance, and Dependence

Many people who are prescribed opioid pain relievers are worried that they may become addicted to these medications. This fear stems from the fact that opioid medications can cause euphoria and pleasure when used by people who are not in pain. However, when these medications are used to treat physical pain, it is extremely unlikely that patients will become addicted to them. Addiction is a psychological need for the drug that very rarely affects people who take opioids for pain control. "Feeling high" from opiates does not happen to people who take them for pain control. People often confuse withdrawal and addiction, but they are different.

Just as a person on long-term opioids stops experiencing some of the negative side effects after a few days, they may also stop getting proper pain relief after taking them for a while. This phenomenon is called tolerance. As patients develop tolerance, they will need higher doses of the opioids to get adequate levels of pain relief. Tolerance is a completely normal aspect of taking opioid painkillers, and is nothing to be concerned about. The point of using these medications is to keep pain well controlled, therefore the exact doses that any patient requires are not important as long as they can be kept comfortable. If you think you need to change the dose, always ask the provider that gave you the prescription.  Do not try to change the dose on your own, as this may cause unwanted side effects.

As a person remains on regular doses of opioids, for as little as a week, their bodies will begin to adapt to the medications. This causes tolerance, but it can also cause dependence. Dependence means that the body "gets used to" the opioids. Dependence DOES NOT equal addiction. Dependence is a natural, physical phenomenon that happens to everyone on long-term opioid therapy. The only important thing to know about dependence is that once a patient becomes dependent on opioids, they will feel very sick if they stop the medication abruptly. Patients who are dependent on opioids and stop them all of the sudden will feel lousy, like they have the flu. This is called withdrawal and the symptoms related to it can start within 2 days of abruptly stopping opioids and may last up to 2 weeks. Withdrawal is preventable if you decrease the opioids slowly, generally over a week or so. The exact amount of time to wean is different based on dose, length of time on opioids, and some other individual factors. It is important to remember that dependence is normal, and happens to everyone who takes opioids for a long period of time. Talk to your provider if you have any concerns. 



If you have questions or concerns about the medication that you have been prescribed, please contact your healthcare team. OncoLink is designed for educational purposes only and is not engaged in rendering medical advice or professional services. The information provided through OncoLink should not be used for diagnosing or treating a health problem or a disease. It is not a substitute for professional care. If you have or suspect you may have a health problem, you should consult your health care provider.


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