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Dr Roger Henderson
Reviewed by Roger HendersonReviewed on 29.04.2024 | 3 minutes read

Senna is a type of over-the-counter medicine known as a stimulant laxative. It is a natural medicine made from the senna plant and is used over the counter to treat short term constipation (difficulty passing stools) in adults and children. Lifestyle measures such as increasing dietary fibre, increasing exercise levels and increasing water intake are usually a good place to start. When wanting to start a laxative, it is important to know there are different types of laxatives based on how they work. Bulk-forming laxatives make the stool bigger by helping them to keep hold of water, osmotic laxatives draw in water to soften the stool, and stimulant laxatives like senna increase the movement of the bowels by acting on the nerves that control the muscles of the bowel.

Doctor’s advice

How do I take it?

Senna is available in tablet and oral liquid forms. Senokot is a branded version of senna. Adults are normally recommended to take 7.5mg - 15mg senna in the evening before bed, this is 1 or 2 tablets (depending on the tablet strength) or 5ml to 10ml liquid.

Senna should only be used in children (ages 17 or younger) under the recommendation of a healthcare professional.

You should not take senna for more than 1 week unless this has been advised by your doctor and you should speak to your doctor if you are still unable to pass a stool within 3 days of using senna.

How does it work?

Senna works by increasing the movement of the large intestine and increasing the amount of water inside it, making it easier to pass a stool. Senna is made up of sennosides which are converted to their active form (medical term - rhein anthrone) by bacteria in the large intestine. This active form irritates intestinal cells (increasing intestinal movements) and increases the secretion of water into and reduces the exit of water from the large intestine. Senna normally takes 8 to 12 hours to work, allowing you to pass a stool the next morning.

Pharmacist recommended products

Who should not take the medication?

Do not take senna if you have previously had an allergic reaction to senna or another ingredient listed in the medication. Senna should not be used for losing weight.

You should speak to your doctor before taking senna if you - have liver or kidney problems, are pregnant or breastfeeding. If you have medical conditions related to your stomach or bowels that you see your doctor for, then it would be best to discuss with your doctor before starting.

Bowel habit is unique to every person, although if you have a change from your normal if would be best to discuss with your doctor. If you have any serious symptoms like bleeding or mucus it is important to mention this to your doctor.

Senna can interact with other medicines such as amitriptyline, citalopram, clarithromycin, digoxin and lithium. If you take any prescription, over-the-counter or herbal medicines, ask your doctor or pharmacist to check they are safe to take alongside senna.

Are there any side-effects?

Side effects can include stomach ache, stomach cramps, diarrhoea (all are more common if you have IBS) and your urine can change colour to be red-brown (but don’t worry, this is harmless).

Seek urgent medical advice if you develop symptoms of an allergic reaction such as a skin rash, shortness of breath, wheezing, tightness in your chest or throat or swelling of your tongue, mouth, lips, face or throat.

People who take senna long term may experience - low potassium levels in the blood, protein in your urine, blood in your urine and the development of a ‘lazy bowel’(where your bowel becomes dependent on senna).

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Dr Roger Henderson
Reviewed by Roger Henderson
Reviewed on 29.04.2024
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