1. About senna
Senna is a natural laxative made from the leaves and fruit of the senna plant. It's used to treat constipation (difficulty pooing).
Senna comes as tablets and as a liquid that you swallow (syrup).
It's available on prescription and to buy from pharmacies. Small packs are available to buy from supermarkets (up to 20 standard strength tablets, 10 maximum strength tablets or 100ml of syrup).
It's also combined with other ingredients in constipation remedies such as Manevac and Senokot Dual Relief tablets.
Before trying senna, it’s better to try other ways to help your constipation by:
- having more fibre in your diet
- drinking more water and exercising
Only use senna if you have tried other types of laxatives first such as:
- "bulk-forming" laxatives like Fybogel (ispaghula husk) or methylcellulose
- "osmotic" laxatives like lactulose or polyethylene glycol
2. Key facts
- Senna takes about 8 hours to work.
- It's best to take senna at bedtime so it works overnight.
- The most common side effects are stomach cramps and diarrhoea. These are usually mild and short-lived.
- Do not take senna for more than 1 week. Long-term use of senna can stop your bowel working properly on its own.
- Pee may turn a red-brown colour while you take senna. This is harmless and returns to normal after treatment has finished.
3. Who can and cannot take senna
Senna is safe to take for most adults aged 18 years and over.
It can only be used by children (aged 17 years or younger) if a doctor or pharmacist recommends it.
Only give senna to children if a doctor or pharmacist recommends it.
Senna may not be suitable for everyone. To make sure senna is safe for you, tell your doctor or pharmacist before starting senna if you:
- have ever had an allergic reaction to senna or any other medicine in the past
- have a bowel obstruction
- have Crohn's disease or ulcerative colitis
- have appendicitis
- are trying to get pregnant, already pregnant or breastfeeding
4. How and when to take it
Take senna once a day at bedtime. You can take senna with or without food.
The normal dose of senna tablets for constipation in:
- adults and children aged 12 years and over is 1 or 2 tablets at bedtime (or 1 tablet of Senokot Max Strength)
- children aged between 6 and 11 years is 1 single tablet at bedtime
The normal dose of senna syrup for constipation in:
- adults and children aged 6 years and over is one or two 5ml spoonfuls at bedtime
- children aged 2 to 5 years (only under medical supervision) is half to one 5ml spoonful at bedtime
Liquid senna comes with a plastic cup or spoon to measure the dose. Do not use a kitchen spoon as it will not give the right amount. If you do not have a cup or spoon, ask your pharmacist for one.
Senna takes about 8 hours to work. It's normal to take it at bedtime so it works overnight.
Drink plenty of fluids (6 to 8 glasses a day) while you are taking senna or your constipation may get worse.
What if I forget to take it?
If you forget a dose of senna, do not worry, just take the next dose the following evening.
Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.
What if I take too much?
Taking an extra dose of senna is unlikely to harm you.
You may get stomach pain and diarrhoea but this should ease off within 1 or 2 days.
If you're worried, talk to your doctor or a pharmacist.
5. Side effects
Like all medicines, senna may cause side effects in some people, but many people have no side effects or only minor ones.
Common side effects
Common side effects, which happen in more than 1 in 100 people, are stomach cramps and diarrhoea. You are particularly likely to get stomach cramps and diarrhoea with senna if you have constipation related to irritable bowel syndrome.
Your pee may turn a red-brown colour while you are taking senna. This is normal and returns to normal after treatment has ended.
Talk to your doctor or pharmacist if the side effects bother you or do not go away.
Serious side effects
A very rare but serious side effect of senna is a severe raised, red, itchy skin rash on any part or all of your body.
If you get a severe skin rash, stop taking senna and call your doctor straight away.
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to senna.
These are not all the side effects of senna. For a full list see the leaflet inside your medicines packet.
7. How to cope with side effects of senna
What to do about:
- diarrhoea – you may need to stop taking senna or reduce your dose. Until the diarrhoea stops, drink lots of fluids, such as water or squash, to avoid dehydration. Signs of dehydration include peeing less than usual or having dark strong-smelling pee. Reducing your dose of senna may also help diarrhoea. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
- stomach cramps – 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. If the cramps do not get better, try reducing your dose of senna. If you are in a lot of pain, speak to your pharmacist or doctor.
8. Pregnancy and breastfeeding
Senna may not be suitable if you're pregnant or breastfeeding.
Constipation is common at the end of pregnancy and just after having a baby. If you're pregnant or breastfeeding, it's better to ease constipation without taking a medicine.
Your doctor or midwife will first advise that you eat more fibre and drink plenty of liquids. You'll also be encouraged to do gentle exercise.
If dietary and lifestyle changes do not work, you may be recommended a laxative. Laxatives are usually safe for pregnant women to take because most of them are not absorbed by the digestive system. This means that your baby will not feel the effects of the laxative.
However senna is partly absorbed by your gut. Your doctor or midwife will usually only recommend senna if other laxatives have not worked.
For more information about how senna can affect you and your baby during pregnancy, read this leaflet on treating constipation on the Best Use of Medicines in Pregnancy (BUMPs) website.
10. Cautions with other medicines
There are some medicines that do not mix well with senna and can change the way it works.
Tell your doctor if you are taking these medicines before starting senna:
- a heart medicine such as digoxin or quinidine
- a diuretic (tablets which make you pee more)
- steroid tablets
- liquorice root preparations
Mixing senna with herbal remedies and supplements
Apart from liquorice root preparations, there are no known problems with taking other herbal remedies and supplements with senna.
11. Common questions about senna
How does senna work?
When will I feel better?
How long will I take senna for?
Is it safe to take senna for a long time?
Can I take different laxatives together?
Are other laxatives any better?
Will it affect my fertility?
Will it affect my contraception?
Is there any food or drink I need to avoid?
Can I drink alcohol with it?
Can I take senna after surgery?
Can lifestyle changes help constipation?
Page last reviewed: 24/01/2019
Next review due: 24/01/2022