- 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 a 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.
Senna is safe to take for most adults.
Do not give senna to a child under 6 years old unless your doctor has said it's OK.
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:
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 - one or two tablets at bedtime (or 1 tablet of Senokot Max Strength)
- children aged between 6 and 11 years - one single tablet at bedtime
The normal dose of senna syrup for constipation in:
- adults and children aged 6 years and over - one or two 5ml spoonfuls at bedtime
- children aged 2 to 5 years (only under medical supervision) - 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 don't 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, don't 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 by accident is unlikely to harm you.
You may get stomach pain and diarrhoea but this should ease off within a day or two.
If you're worried, talk to your doctor or a pharmacist.
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 don't 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.
Call 999 or go to A&E if:
- you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
- you're wheezing
- you get tightness in the chest or throat
- you have trouble breathing or talking
- your mouth, face, lips, tongue or throat start swelling
You could be having a serious allergic reaction and may need immediate treatment in hospital.
These are not all the side effects of senna. For a full list see the leaflet inside your medicines packet.
You can report any suspected side effect to the UK safety scheme.
What to do about:
- diarrhoea - 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 don't get better, try reducing your dose of senna. If you are in a lot of pain, speak to your pharmacist or doctor.
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 don't work, you may be recommended a laxative. Laxatives are usually safe for pregnant women to take because most of them aren't absorbed by the digestive system. This means that your baby won't 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 haven't worked.
Lactulose and Fybogel are safer laxatives to take during pregnancy and while breastfeeding.
For more information about how senna can affect you and your baby during pregnancy, read this leaflet on the Best Use of Medicines in Pregnancy (BUMPS) website.
Tell your doctor if you're:
- trying to get pregnant
There are some medicines that don't 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.
For safety, tell your doctor or pharmacist if you're taking any other medicines, including herbal remedies, vitamins or supplements.
How does senna work?
Senna stimulates the muscles that line your gut, helping them to move poo along your bowel to your anus. It's a stimulant laxative.
When will I feel better?
Senna takes about 8 hours to work.
Talk to your doctor if you are still constipated after 3 days.
How long will I take senna for?
Take senna for a few days only and certainly no longer than a week. If you take senna for longer, your bowel can start to rely on it, rather than working on its own.
If you are still constipated after taking senna for 3 days, talk to your doctor.
Is it safe to take senna for a long time?
Ideally, you should only use senna occasionally and for a few days at a time.
Using senna for longer can lead to prolonged diarrhoea. It can also cause an electrolyte imbalance, where levels of substances like sodium, potassium and magnesium in your body get too high or too low. A severe electrolyte imbalance can cause serious health problems such as muscle spasm and twitching, and even convulsions.
Using senna for many weeks, even months, could also stop your bowel working properly on its own.
Can I take different laxatives together?
For most people, 1 laxative will be enough to relieve constipation.
Occasionally, you may need to take 2 different types of laxatives at the same time to get your bowels moving again. Only take 2 laxatives together on the advice of your doctor or pharmacist as there is an increased risk of side effects.
Are other laxatives any better?
There are other types of laxatives. They work in a different way to senna but are equally good at treating constipation. Some take longer to work than senna.
- Bulk-forming laxatives, for example Fybogel and methylcellulose. These increase the "bulk" or weight of poo which in turn stimulates your bowel. These laxatives take 2 or 3 days to work.
- Osmotic laxatives, for example lactulose. These draw water from the rest of the body into your bowel to soften poo and make it easier to pass. They take at least 2 days to work.
- Surface-wetting laxatives, for example arachis oil and docusate sodium. These let water into poo to soften it and make it easier to pass.
Will it affect my fertility?
There's no firm evidence to suggest that taking senna will reduce fertility in either men or women.
However, speak to a pharmacist or your doctor before taking it if you're trying to get pregnant.
Will it affect my contraception?
If you have severe diarrhoea your contraceptive pills may not protect you from pregnancy. Look on the pill packet to find out what to do.
Is there any food or drink I need to avoid?
You can eat and drink normally with senna.
It might be a good idea to stop eating pastries, puddings, sweets, cheese and cake for a while as these foods can make constipation worse.
Can I drink alcohol with it?
Yes, you can drink alcohol with senna.
Can I take senna after surgery?
It's quite common to have constipation after surgery. Using a laxative may help relieve the discomfort.
If you have constipation after an operation, it's better to use lactulose because it's gentler on your stomach muscles than senna.
Can lifestyle changes help constipation?
It's often possible to relieve constipation without having to use laxatives. Before trying senna - or to stop constipation coming back - it may help to:
- get more fibre into your diet - aim for about 30g of fibre a day. High-fibre foods include fruit, vegetables and cereals. If you aren't used to a high-fibre diet, it may be best to increase the amount of fibre you eat gradually
- add bulking agents, such as wheat bran, to your diet. These will help make your poo softer and easier to pass (although bran and fibre can sometimes make bloating worse)
- drink plenty of water - to keep poo soft
- exercise - keeping your body active will help to keep your gut moving