Laxatives are not one thing. There are four main types, they work differently, and their side effects differ accordingly. Choosing the wrong type for your situation is the usual reason someone finds a laxative unhelpful or unpleasant.
- Bulk-forming such as psyllium husk. Add fibre and draw in water. Work over a day or two and need plenty of fluid to work safely. See psyllium husk side effects.
- Osmotic such as lactulose or macrogol. Draw water into the bowel. Cause bloating and wind at first.
- Stimulant such as senna or bisacodyl. Make the bowel contract. Fast and effective, and the type most associated with cramping and with problems from prolonged use.
- Stool softeners, which make stools easier to pass rather than stimulating the bowel.
Common side effects of this pharmacy medicine or category
- Abdominal cramping, most with stimulant laxatives. See laxatives causing cramps.
- Bloating and wind, most with bulk-forming and osmotic types
- Diarrhoea if the dose is too high
- Nausea
- Dehydration and salt imbalance with prolonged or excessive use
Bulk-forming laxatives need adequate fluid. Taken without enough water they can worsen the problem rather than solve it, and rarely cause an obstruction. This is the most common mistake with this type.
Interactions and duplicate-ingredient risks
Stimulant laxatives are intended for short-term use. Taken daily over long periods they can lead to reliance on them for a bowel movement, and prolonged use is associated with salt imbalance. If you have been using one most days for weeks or months, that is worth reviewing rather than continuing, and stopping is usually done gradually alongside a different approach.
Bulk-forming laxatives can reduce absorption of other medicines taken at the same time, so separate them by a couple of hours.
Constipation is frequently caused by another medicine, and treating it with a laxative without identifying that is a partial fix. Common culprits include codeine and other opioids, iron tablets, some antidepressants and antihistamines, and some blood pressure medicines. See medicines causing constipation.
Children, pregnancy, older adults and chronic medicine cautions
- Children. Ask a pharmacist rather than using an adult product; constipation in children is managed differently.
- Pregnancy. Constipation is common in pregnancy, and some laxatives are preferred over others. Ask first.
- Older adults. More vulnerable to dehydration and salt imbalance, and more likely to have a medicine causing the constipation.
- Kidney or heart conditions, where some laxatives are less suitable.
Warning signs and overdose concerns
Do not treat these with a laxative. See a doctor:
- Blood in the stool, or black tarry stools
- Unexplained weight loss
- A persistent change in your usual bowel habit, particularly over the age of 50
- Severe abdominal pain, vomiting, or no bowel movement at all with a swollen abdomen
- Constipation alternating with diarrhoea
Get emergency help for severe abdominal pain with vomiting and no bowel movement, which can indicate an obstruction.
Questions to ask a pharmacist before using it
- Which type suits my situation?
- Could one of my medicines be causing this?
- How long can I use it before seeing a doctor?
- How much fluid do I need with it?
- I have been using one daily for months. How do I come off it?
Related OTC and medicine-safety guides
- OTC medicine side effects
- Digestive medicine side effects
- Over-the-counter medicine interactions
- When to see a doctor about an OTC medicine
- Report an OTC medicine side effect
See also fibre supplement side effects.
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.