Like all medicines, fentanyl can cause side effects in some people but many people have no side effects or only minor ones.
You're more likely to get side effects with higher doses of fentanyl.
Common side effects
These common side effects of fentanyl happen in more than 1 in 100 people. There are things you can do to help cope with them:
Constipation
Get more fibre into your diet such as fresh fruit, vegetables and cereals, and drink plenty of water. If you can, it may also help to do some gentle exercise. If this does not help, talk to your pharmacist or doctor.
It's likely you'll need to take a laxative to help prevent or treat constipation caused by fentanyl. Ask your doctor if you're not taking a laxative or if your constipation symptoms do not go away.
Feeling or being sick (nausea or vomiting)
This side effect should normally wear off after a few days. Stick to simple meals and do not eat rich or spicy food.
If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
Ask your doctor about anti-sickness medicine if it gets worse or lasts longer than a few days.
If you take the combined contraceptive pill or the progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Stomach pain
Try to rest and relax. It can help to eat and drink slowly and have smaller and more frequent meals.
Putting a heat pad or covered hot water bottle on your stomach may also help. But do not do this if it would be near to your fentanyl patch. Patches should not be exposed to heat.
If you're in a lot of pain, speak to your pharmacist or doctor.
Feeling sleepy or tired
These side effects should wear off within a week or 2 as your body gets used to fentanyl. Talk to your doctor if they carry on for longer.
Do not drink alcohol as it will make these side effects worse. Do not drive, ride a bike or use tools or machinery.
Feeling dizzy or a sensation of spinning (vertigo)
Stop what you're doing and sit or lie down until you feel better. Do not drink alcohol as it will make these side effects worse.
If the feeling does not go away, do not take any more medicine and speak to a pharmacist or your doctor.
Do not drive, ride a bike or use tools or machinery.
Confusion
Talk to your doctor if you feel confused. Your dose may need to be changed.
Headaches
Make sure you rest and drink plenty of fluids. Do not drink alcohol while taking fentanyl as this can make headaches worse. Ask your pharmacist to recommend a painkiller.
Headaches should usually go away after the first week of taking fentanyl. Talk to your doctor if they last longer than a week or are severe.
Itching or skin rashes
It may help to take an antihistamine, which you can buy from a pharmacy. However, some types of antihistamine may not be suitable to take with fentanyl. Check with the pharmacist to see what type is suitable for you.
If symptoms do not go away or it gets worse talk to your doctor as you may need to try a different painkiller.
Speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.
Serious side effects
Serious side effects happen in less than 1 in 100 people.
Call a doctor or call 111 now (and remove a patch if you're wearing one) if:
- your muscles feel stiff for no obvious reason
- you feel dizzy, tired or have low energy – these could be a sign of low blood pressure
Immediate action required: Call 999 now if:
- you have difficulty breathing or have short shallow breathing
- you've had a fit or seizure
Do not drive yourself to A&E.
The person you speak to at 999 will give you advice about what to do.
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to fentanyl.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Do not drive yourself to A&E.
The person you speak to at 999 will give you advice about what to do.
Long-term side effects
Some people with conditions that need long-term pain relief may need to take fentanyl for a long time.
If you need to take it for a long time your body can become used to it (known as tolerance). That means you need higher doses to control your pain over time.
Some people can become more sensitive to pain (hyperalgesia). If this happens, your doctor will reduce your dose gradually to help these symptoms.
It's possible to become addicted to fentanyl. For this reason, your dose will be reviewed to make sure you're only having the amount you need to control your pain.
Your treatment plan may include details of how and when you'll stop fentanyl.
If you're having treatment for cancer pain or other severe pain, your pain control will be carefully monitored. Speak to your doctor if you're worried about tolerance, hyperalgesia or becoming addicted.
Other side effects
These are not all the side effects of fentanyl. For the full list, see the leaflet inside you medicines packet.
Reporting side effects
You can report any suspected side effect using the Yellow Card safety scheme.
Visit the Yellow Card website for more information about reporting side effects.