Neil asked this a while back http://www.thenakedscientists.com/forum/index.php?topic=9120.0 but there were no 'sensible' replies.
Well I never.
Anyway, this is wht BUPA have to say:
How do laxatives work?
By swelling up within your intestines, bulk-forming laxatives soften and increase the volume of your stools. This then encourages your bowels to move and push the stools out. The full effect of this type of laxative can take a few days to occur.
Stimulant laxatives work by speeding up the movements of the intestines. They take 8-12 hours to work. Osmotic laxatives work by reducing the amount of water absorbed from the bowel and so increasing the amount of water in your stools. This makes them softer and easier to pass.
What are they for?
Laxatives are used to help relieve constipation. But, before taking a laxative, you need to be sure you really are constipated. Normal bowel movements vary from person to person. Some people usually go at least once a day, while for others once every two or three days is normal.
A good guide to whether you are really constipated is that your stools have become hard and you are passing them less often than usual - or having difficulty passing them. It is important to remember that over-use of laxatives can cause the bowels to become 'lazy' and not work properly. If you are in reasonably good health, you should not need to take laxatives on a regular basis. A balanced diet that includes fibre, such as fruit, vegetables and whole grain cereals, and an adequate fluid intake (see self-help below) should keep your bowel movements regular. If you do get constipated, it's worth trying to increase your fibre intake and drink more water before taking laxatives.
In general, laxatives should be reserved for people who are at risk of an illness, such as angina, being made worse by straining to pass stools, or for people who have piles (haemorrhoids). Laxatives can also be helpful for people with constipation caused by prescription drugs (e.g. codeine) or by illness; in older people who are constipated because of reduced mobility; and to help get rid of worms. They are also prescribed to clear the digestive tract before surgery or certain tests. It is particularly important not to give laxatives to children unless a doctor has prescribed them.
Bulk-forming laxatives are particular helpful for people with small hard stools who can't increase the amount of natural fibre they eat. This includes people with colostomy or ileostomy bags, piles, irritable bowl syndrome, and anal fissures (small cracks around the anus). Stimulant laxatives are helpful if the bowel is full and a bulk-forming laxative is inappropriate. Some osmotic laxatives are best used when urgent emptying of the bowels is needed.
Common Side effects with the bulk-forming laxatives are wind and a swollen abdomen. They can also cause blockage of the intestine (see warning below). Stimulant laxatives can cause stomach pains and cramping, and too large a dose can cause diarrhoea. Osmotic laxatives can cause wind, cramps and discomfort in the abdomen.
Use laxatives with care if ...
You should use a laxative with care if you:
have a narrowed intestine, or one that isn't working properly
are elderly or debilitated (some laxatives can alter the balance of fluid and important body chemicals such as sodium and potassium. People who are frail or ill may be unable to adjust to these changes)
have kidney or liver disease (especially with laxatives containing magnesium salts, as an excess of magnesium can build up in the body if these organs are not working properly)
are pregnant or breast feeding (especially with macrogols - see list of common laxatives below)
When to avoid laxatives
If you are sensitive to gluten or have coeliac disease, avoid wheat bran.
If you have an inflammatory diseases of the intestines such as ulcerative colitis laxatives, particularly macrogols, should be avoided.
Interactions with other medicines
Do not take any other medicines or herbal remedies with a laxative, including those you have bought without a prescription, before talking to your doctor or pharmacist.
How to use a laxative
Bulk-forming laxatives come as powders, granules or tablets. They must be taken with plenty of fluids (see warning below). Unprocessed wheat bran (Trifyba) taken with food or fruit juice is a very effective bulk-forming laxative.
Stimulant laxatives come as tablets, suppositories, capsules, liquids and enemas. Senna has been used for centuries as a stimulant laxative. Osmotic laxatives are available as powders, liquids or enemas.
Laxatives of each of the three basic types can be bought without a prescription. Laxatives should only be used for short periods of time, until your normal bowel movements are re-established. Then, you should keep your bowel movements regular by eating a balanced diet with plenty of fluids and fibre. When you are taking a stimulant laxative, start with a low dose and build up gradually until you feel comfortable when you pass a stool.
Warning: If you take a bulk-forming laxative, drink plenty of fluids to help ensure the intestines do not become blocked.