Laxatives Compared: Which One and When
The four classes, what each actually does, and how the choice follows from whether the stool is too hard or simply not moving.
Part of the Complete Constipation Guide.
Key fact: Laxatives are chosen by what is wrong with the stool, not by strength. Hard stool needs softening; soft stool that still will not come needs stimulating. Getting that the wrong way round is why a laxative "does not work".
The four classes
| Class | Examples | What it does | Typical onset |
|---|---|---|---|
| Bulk-forming | Ispaghula husk (Fybogel), methylcellulose, sterculia | Holds water in the stool, increasing bulk and softness | 2–3 days |
| Osmotic | Macrogol (Laxido, Movicol), lactulose, magnesium salts | Draws water into the bowel to soften stool | 1–3 days (macrogol often faster) |
| Stimulant | Senna, bisacodyl, sodium picosulfate | Increases bowel muscle activity to move stool along | 6–12 hours |
| Softeners and outlet treatments | Docusate, glycerol suppositories, mini and retention enemas | Soften or lubricate stool, or act directly at the rectum | 15 minutes to a few hours |
Bulk-forming
First line for most people with ordinary constipation. Ispaghula husk is soluble fibre that holds water, making the stool larger, softer and easier to move.
- Typical use: one sachet twice daily, stirred into water and drunk promptly
- Fluid is not optional. Bulk-forming laxatives taken without enough fluid make stools bulkier but not softer, which is worse than not taking them
- Give it time. Two to three days, sometimes longer, before judging it
- Expect some wind and bloating in the first week or two while the gut adapts
- Do not take it immediately before bed, and do not take it if you have difficulty swallowing
Bulk-forming laxatives are not used for opioid-induced constipation, and are avoided where there is any suspicion of obstruction or impaction. Adding bulk to a bowel that cannot move it makes matters worse rather than better. See opioid-induced constipation and faecal impaction.
If you have IBS with constipation, fibre behaves differently and insoluble fibre often makes symptoms worse — fibre and IBS covers the distinction, and an ispaghula-based product combined with an antispasmodic, such as Fybogel Mebeverine, exists for exactly that situation.
Osmotic
Osmotic laxatives pull water into the bowel. They are the step taken when stools remain hard despite fibre and fluid.
| Macrogol | Lactulose | |
|---|---|---|
| Form | Sachets of powder dissolved in water | Liquid |
| Typical adult dose | 1–3 sachets daily in divided doses | Titrated to response |
| Wind and bloating | Less | More — it is fermented by gut bacteria |
| Generally preferred? | Yes, in most current UK guidance | Still widely used, particularly in pregnancy |
Macrogol is the one most often taken incorrectly — too little water, too few sachets, stopped after two days. The macrogol guide covers it properly.
Stimulant
Stimulants make the bowel wall contract more actively. They are added when the stool is soft but still will not come, or when emptying feels incomplete.
- Senna, typically 2 to 4 tablets at night
- Bisacodyl, typically 5 to 10mg at night
- Onset is 6 to 12 hours, which is why they are taken at bedtime for a morning effect
- Cramping is the main side effect, and is dose-related
- Not first line on their own, and not for hard stool — stimulating the bowel to push a rock is painful and ineffective
Softeners, suppositories and enemas
- Docusate acts as both a softener and a weak stimulant. It is often used where straining must be avoided, and in opioid-induced constipation
- Glycerol suppositories work at the rectum within about 15 to 30 minutes. Useful when stool is present and ready but will not pass — an outlet problem rather than a transit problem
- Mini enemas and retention enemas are used when oral treatment has not cleared a loaded rectum
These matter more than their reputation suggests. If the problem is at the very end of the process, an oral laxative has to travel the whole bowel to address something sitting two inches from the exit.
The order they are used in
| Step | What it involves |
|---|---|
| Bulk-forming, with fluid | Alongside the dietary and routine changes, not instead of them. |
| If stools stay hard: add or switch to an osmotic | Macrogol generally first. Titrate the dose rather than taking a fixed amount. |
| If stools are soft but will not come: add a stimulant | Senna or bisacodyl at night. |
| If it is an outlet problem: a suppository or mini enema | Particularly where stool is felt in the rectum but cannot be passed. |
| If none of this works: get assessed | Constipation that does not respond to treatment is itself a reason for review — see when it is not just constipation. |
Choosing by what the stool is doing
| What is happening | What it points to |
|---|---|
| Hard, dry, pellet-like stool | Softening problem — bulk-forming, then osmotic |
| Soft stool but nothing moves for days | Transit problem — add a stimulant |
| Soft stool sitting at the outlet, straining without result | Outlet problem — suppository, and consider pelvic floor assessment |
| Watery leakage with a blocked feeling | Possible impaction with overflow — do not take anti-diarrhoeals |
| Constipation with pain that eases after opening bowels | Consider IBS-C rather than simple constipation |
How long, and how to stop
Most laxatives work within about three days. For an acute episode a short course is enough. For chronic constipation, treatment often runs for weeks or months.
- Titrate to stool form, aiming for a soft, formed stool passed without straining — not for a bowel movement every day at all costs
- Do not stop abruptly. Reduce gradually, typically after two to four weeks of comfortable, regular bowel movements
- Expect to keep the lifestyle measures going after the laxative stops, or the pattern returns
Situations that change the answer
- Opioid painkillers — skip bulk-forming entirely; an osmotic plus a stimulant is the standard combination. More here
- Pregnancy — bulk-forming first, then lactulose or macrogol; stimulants only on advice. More here
- Suspected impaction — a different, higher-dose regimen, and bulk-forming is avoided
- Haemorrhoids or an anal fissure — keeping stool soft is part of the treatment, not an optional extra. Stopping piles coming back
- Swallowing difficulty, or fluid restriction — bulk-forming laxatives are generally avoided
Three things that are not true
- "Laxatives make your bowel lazy." Bulk-forming and osmotic laxatives do not cause dependence. The older concern related to prolonged high-dose stimulant use, and even there the evidence is weaker than the reputation. Under-treating chronic constipation causes more harm than treating it
- "Stronger is better." Strength is not the axis. A stimulant is not a stronger bulk-forming laxative; it does a different job, and it is the wrong job if the stool is hard
- "You need to go every day." Three times a day to three times a week is normal. Chasing a daily movement leads to over-treatment and, occasionally, to diarrhoea being mistaken for success
Frequently Asked Questions
Which laxative works fastest?
Stimulant laxatives such as senna and bisacodyl act within 6 to 12 hours, which is why they are taken at night. Glycerol suppositories act at the rectum within about 15 to 30 minutes. Bulk-forming and osmotic laxatives are slower, typically taking one to three days, and are the ones used first.
What is the difference between a bulk-forming and an osmotic laxative?
A bulk-forming laxative such as ispaghula husk is fibre that holds water in the stool, so it needs plenty of fluid alongside it. An osmotic laxative such as macrogol or lactulose draws water into the bowel from the body. Bulk-forming is usually tried first; osmotic is added or substituted if stools remain hard.
Do laxatives make your bowel lazy?
Bulk-forming and osmotic laxatives do not cause dependence. The concern historically related to prolonged high-dose stimulant use, and the evidence for it is weaker than its reputation. Leaving chronic constipation untreated causes more problems than treating it does.
Which laxative should I take with codeine?
Not a bulk-forming one. Opioid-induced constipation is treated with an osmotic laxative and a stimulant together, such as lactulose or macrogol with senna. Bulk-forming laxatives add volume to a bowel that is not moving it along, which can make things worse.
How long can you take laxatives for?
An acute episode needs only a short course. Chronic constipation often needs treatment for weeks or months, titrated to stool form. Laxatives should be reduced gradually rather than stopped abruptly, typically after two to four weeks of comfortable, regular bowel movements.
References
- NHS. Constipation. nhs.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summary: Constipation. cks.nice.org.uk
- Electronic Medicines Compendium. Movicol 13.7g powder for oral solution: Summary of Product Characteristics. medicines.org.uk
- National Institute for Health and Care Excellence. Suspected cancer: recognition and referral (NG12). nice.org.uk
- National Institute for Health and Care Excellence. Quantitative faecal immunochemical testing to guide referral for colorectal cancer in primary care (DG56). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. A persistent change in bowel habit, rectal bleeding or unexplained weight loss should always be assessed rather than treated as constipation. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.