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Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 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 to treat short-term constipation (difficulty passing stools) in adults and children. Lifestyle measures such as increasing dietary fiber, increasing exercise levels, and increasing water intake are usually good places to start. When starting a laxative, it is important to be familiar with different types of laxatives based on how they work. Bulk-forming laxatives make the stool bulkier 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 form. Senokot is a branded version of senna. Adults are normally recommended to take 8.6 mg – 17.2 mg senna in the evening before bed; this is 1 or 2 tablets (depending on the tablet strength).

Senna can be used in children as young as 2 years; follow the dosing directions on the package.

You should not take senna for more than 1 week unless your doctor has advised this, 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 the water inside, 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 water exit, from the large intestine. Senna normally takes 6 to 12 hours to work, allowing you to pass a stool the next morning.

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 are 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 it with them before starting.

Bowel habits are unique to every person, although if you have a change from your normal, it would be best to discuss this with your doctor. If you have any serious symptoms like bleeding or mucus, mention them 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 if they are safe to take alongside senna.

Are there any side effects?

Side effects can include stomach ache, stomach cramps, and diarrhea (all are more common if you have IBS), and your urine can change color to 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 the urine, blood in the urine, and the development of a ‘lazy bowel’ (where your bowel becomes dependent on senna).

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Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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