Senna is a natural laxative made from the leaves and fruit of the senna plant. It's used to treat difficulty pooing (constipation). It works by stimulating the muscles in your gut.
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
Senna comes as tablets and as a liquid that you swallow (syrup).
It's available on prescription and to buy from pharmacies. You can buy small packs 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.
Who can take senna
Most adults aged 18 years and over can take senna.
Young people aged 12 to 17 years can take senna if a doctor or pharmacist says it's OK.
Children aged 11 years and under can take senna if a doctor prescribes it.
Only give senna to a child if a doctor or pharmacist recommends it.
Senna may not be suitable for some people. To make sure it's safe for you, tell your doctor or pharmacist before starting to take senna if you:
- have ever had an allergic reaction to senna or any other medicine
- have signs of dehydration – these include peeing less than usual or having dark, strong-smelling pee
- have severe stomach pain and you are feeling or being sick (nausea or vomiting)
- have a blockage in your bowel (intestinal obstruction)
- have Crohn's disease or ulcerative colitis
- have a serious problem in your stomach (abdomen) such as appendicitis
- have kidney or heart problems
- are trying to get pregnant, already pregnant or breastfeeding
The usual dose of senna tablets and liquid for constipation is:
- adults and children aged 12 years and over – 1 or 2 tablets (7.5mg or 15mg) at bedtime (or 1 tablet of Senokot Max Strength)
- adults and children aged 12 years and over – one or two 5ml spoonfuls (7.5mg or 15mg) of syrup at bedtime
If you have not taken senna before, start with one 7.5mg tablet or one 5ml spoonful of syrup. If that does not work you can increase the dose to a maximum of 2 tablets or two 5ml spoonfuls at bedtime.
Only give senna that you buy to a young person aged 12 to 17 years if a doctor or pharmacist recommends it.
The usual starting dose is:
- adults aged 18 years and over – two 7.5mg tablets, taken once a day at bedtime
- children aged 6 to 17 years – one 7.5mg tablet, taken once a day at bedtime
A doctor may prescribe higher doses for adults and children up to a maximum of 6 tablets (30mg) taken once a day if necessary.
Dosage for senna syrup prescribed by a doctor
The usual dose is:
- adults aged 18 years and over – one or two 5ml spoonfuls (7.5mg or 15mg) at bedtime
- children aged 4 to 17 years – half a 5ml spoonful to two 5ml spoonfuls at bedtime
- childen aged one month to 3 years – half a 5ml spoonful
Higher doses of up to six 5ml spoonfuls are sometimes needed in adults and in children aged 4 years and over. The usual maximum dose for younger children is two 5ml spoonfuls taken once a day at bedtime.
Senna syrup comes with a plastic cup or spoon to measure the dose. Do not use a kitchen spoon as it will not measure the right amount.
How to take it
Take senna once a day at bedtime. You can take it with or without food.
Senna takes about 8 hours to work. It's usual to take it at bedtime so it works overnight.
Drink plenty of fluids (6 to 8 glasses a day) while you're taking senna or your constipation may get worse.
How long to take it for
Take senna for a few days only and no longer than a week. If you take it for longer, your body can start to rely on it, rather than your bowels doing the work on their own.
If you are still constipated after taking senna for 3 days, talk to your doctor.
If you forget to take it
If you forget a dose of senna, just skip the missed dose and 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.
If you often forget doses, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.
If you 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.
Common side effects
Common side effects of senna, 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 (IBS).
Stop taking senna if you get diarrhoea.
If you take contraceptive pills and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Your pee may turn a red-brown colour while you’re taking senna. This is harmless 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.
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to senna.
Ideally you'll only take senna for a few days. Taking it for a long time can 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 spasms or twitching, and even cause a seizure or fit.
Using senna for many weeks, even months, could also stop your bowel working properly on its own.
Only take senna regularly long term if it has been prescribed for you.
Other side effects
These are not all the side effects of senna. For a full list see the leaflet inside your medicines packet.
Senna and pregnancy
If you're pregnant you may try and treat constipation without taking a medicine. You can try eating more fibre and drinking plenty of fluids. It may also help to do gentle exercise.
Senna is occasionally used in pregnancy, and there is no evidence that it causes harm. However, it's better to try other types of laxative first because there is more information to say that they're safe.
If your doctor or midwife says your baby is healthy you can take senna while you are breastfeeding.
Only tiny amounts pass into breast milk which are unlikely to cause any side effects in your baby.
Although you could try treating constipation without taking a laxative, or taking a different laxative such as lactulose or Fybogel, it's important that you take the medicine that works for you. Talk to your doctor or pharmacist who will help you decide what's best for you.
If your baby is not feeding as well as usual, or has diarrhoea, then talk to your health visitor, midwife, pharmacist or doctor.
Senna and fertility
There's no evidence to suggest that taking senna reduces fertility in either men or women.
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 any of these medicines before starting to take senna:
- tablets that make you pee more (diuretics)
- steroid tablets such as prednisolone
- heart medicines such as digoxin
These medicines can upset the balance of salts and minerals in your body if you have too much senna.
If you are taking digoxin, this imbalance makes it more likely you will have the serious side effects of digoxin.
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 if your doctor or pharmacist recommends it, as you're more likely to get side effects.
Mixing senna with herbal remedies and supplements
Tell your doctor if you're taking any remedies containing liquorice root.
It is not possible to say if herbal remedies and supplements are safe to take with senna. They're not tested in the same way as pharmacy and prescription medicines.
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