If you haven’t been to the toilet for a few days, the right choice is almost never the fastest laxative. There are four families of laxative, they work by different mechanisms, and they take anywhere from twenty minutes to three days to act. Choosing well means matching the mechanism to your situation: constipation that has been building for weeks needs something that softens stools steadily, not a stimulant that forces one bowel movement tonight and leaves you back where you started tomorrow.
First: check that it is constipation
The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) defines it as fewer than three bowel movements a week, stools that are hard, dry or pellet-like, movements that are difficult or painful, or the feeling of not having finished. There is no universal “normal” frequency: some people go three times a day and others three times a week, and both are fine. What matters is the change from your own pattern, and the difficulty passing.
It is commoner than it looks. According to the NIDDK, around 16 in 100 adults have symptoms of constipation, rising to 33 in 100 from the age of 60.
Before buying anything, check the obvious: water, dietary fibre, movement, and whether you have started a new medicine. Plenty of everyday drugs cause constipation — opioids, antimuscarinics, iron, some antacids — and in that case a one-off laxative won’t fully solve the problem.
The four families, and how long each takes
| Type | Common examples | How it works | How long it takes |
|---|---|---|---|
| Bulk-forming | Ispaghula husk (Plantago ovata), bran | Holds water and increases stool volume | 12-24 h; peak effect sometimes not until 2-3 days |
| Osmotic: macrogol | Macrogol 3350 sachets | Holds water in the bowel by osmosis | 24-48 h |
| Osmotic: lactulose | Lactulose solution or sachets | Acts on reaching the colon, fermented by gut flora | 1-2 days |
| Stimulant | Bisacodyl, senna | Stimulates movement of the colon | 6-12 h orally; around 20 min as a suppository |
| Fast-acting rectal | Glycerol suppositories | Softens stool and mildly irritates the rectum | Minutes |
That table explains the most frequent mistake. Someone who has had hard stools for three weeks buys bisacodyl because “that’s the one that works”, has a forced bowel movement that night, and is back to square one two days later, because nothing has changed about the consistency of the stool. What that case needs is a bulk-forming agent or macrogol for several days in a row.
Where to start, depending on your case
Ongoing constipation, hard stools, a low-fibre diet. Start with a bulk-forming agent such as ispaghula husk (Plantago ovata). It is soluble fibre that absorbs up to 40 times its weight in water: it increases stool volume, reduces viscosity and speeds up transit. The adult dose for constipation is 7 to 11 g a day, usually one sachet two or three times daily. It is the only group the product information accepts for prolonged use, precisely because it acts locally and forces nothing.
Constipation that fibre doesn’t shift, or very hard stools. Macrogol 3350 is the next step up. It holds water by osmosis, softens the stool, and is not absorbed. In adults, 1 to 3 sachets a day, each dissolved in 125 ml of water. A normal course shouldn’t run beyond two weeks, though it can be repeated.
Pregnancy or breastfeeding. Both ispaghula and macrogol 3350 can be used in either situation according to their product information, because systemic exposure is negligible. Even so, mention it to your midwife or doctor first.
A one-off, uncomfortable episode with stool already in the rectum. Here the rectal route does make sense: a glycerol suppository softens the stool and triggers the urge to go within minutes. The general pattern is one suppository a day and no more than seven days in a row.
The three mistakes that cancel out the treatment
Taking ispaghula with too little water. This is the most dangerous on the list. Fibre that isn’t properly hydrated can block the throat or the oesophagus, and in the bowel it can cause distension, obstruction or faecal impaction. The right way: dissolve the sachet in a glass of cold water (200 ml), drink it immediately, and follow it with another glass of fluid, keeping to one or two litres of water a day. And don’t lie down straight afterwards. If you have difficulty swallowing or any condition of the oesophagus, this group is contraindicated.
Taking bisacodyl with milk, an antacid or omeprazole. Bisacodyl tablets are gastro-resistant: the coating is designed not to dissolve until it reaches the colon, which is where the drug needs to act. Anything that reduces stomach acidity can dissolve that coating early, and the result is abdominal cramps. The product information recommends leaving at least two hours between the tablet and milk or those medicines. Tablets are swallowed whole, not chewed, with plenty of fluid, at night: taken that way, the bowel movement comes roughly ten hours later.
Taking other medicines at the same time as the laxative. Macrogol speeds up transit and can temporarily reduce the absorption of whatever else is on board; its product information says not to take other oral medicines from one hour before to one hour after. Ispaghula, for its part, can delay the absorption of minerals such as calcium, iron and zinc, of vitamin B12, and of coumarin anticoagulants: leave one to two hours between it and any other drug.
Stimulants are not for everyday use
Bisacodyl and senna are indicated for occasional constipation, not as a routine. Prolonged use of stimulant laxatives causes loss of water, potassium and other salts, and that low potassium amplifies the action of digoxin and interferes with antiarrhythmics such as quinidine. The risk grows if you are also taking thiazide diuretics or corticosteroids, which push potassium the same way.
Two things worth being clear about. Stimulant laxatives do not help you lose weight: they act in the colon, by which point calories and nutrients have already been absorbed in the small intestine. And they should not be taken daily or for long stretches without investigating why you are constipated.
One detail that gets overlooked: macrogol combined with electrolytes is high in sodium. One sachet supplies the equivalent of 9.3% of the WHO’s recommended maximum daily sodium intake, and the maximum daily dose on prolonged treatment reaches 28%. If you are on a low-salt diet, say so.
When to see a doctor
Stop self-treating and get advice if any of these appear:
- Blood in the stool or rectal bleeding with no known cause.
- Severe abdominal pain, nausea or vomiting alongside the constipation: these can indicate appendicitis or bowel obstruction, and laxatives are contraindicated in that context.
- You can’t pass a stool after using a laxative, or nothing happens 12 hours after the maximum daily dose.
- Symptoms persist beyond 7 days of treatment with a laxative.
- An unexplained change in bowel habit, particularly from the age of 50, or alternating constipation and diarrhoea.
- Unintentional weight loss, fever or marked fatigue alongside it.
- Constipation in a child under 12 that diet doesn’t resolve: most of these laxatives are not recommended at those ages without medical supervision.
- Signs of dehydration while taking laxatives: intense thirst, passing very little urine, weakness.
This information is general and does not replace assessment by a healthcare professional. Always read the leaflet that comes with your medicine and ask your pharmacist or doctor if anything is unclear. Sources: Definition & Facts for Constipation (NIDDK, NIH) · Product information, Plantago ovata 3.5 g oral powder (CIMA, AEMPS, in Spanish) · Product information, Movicol powder for oral solution (CIMA, AEMPS, in Spanish) · Product information, DulcoLax Bisacodyl 5 mg gastro-resistant tablets (CIMA, AEMPS, in Spanish) · Product information, Lactulosa Lainco 3.33 g/5 ml oral solution (CIMA, AEMPS, in Spanish) · Product information, Glycerol Suppositories Cuve for Adults (CIMA, AEMPS, in Spanish)