There are four main types of laxative, so it depends which one you take. ‘Bulk-forming’ laxatives (e.g. methylcellulose or ispaghula husk) are usually the first to be recommended. These contain fibre, which absorbs water and makes the poo larger. This stretches the bowel wall, which stimulates it to contract and move the poo along. The increased water content also makes the poo softer, so it’s easier to pass.
If that doesn’t work, you might try an ‘osmotic’ laxative (e.g. lactulose or polyethylene glycol), which work via the process of osmosis, where water moves from a high water concentration to a low one. These laxatives contain molecules such as sugars and salts that lower the water concentration in the bowel, causing water to enter and soften the poo. There are also ‘poo-softening’ laxatives (e.g. arachis oil, docusate sodium), which work by reducing the poo’s surface tension so that more water can seep in.
Finally, if you’re still constipated, you might need a ‘stimulant’ laxative (e.g. senna, bisacodyl). These work directly on the nerves that control the muscles in the bowel wall, causing the bowel to contract. These laxatives work faster than the others – within about 6 to 12 hours.
Talk to your pharmacist (or GP) if a certain type of laxative isn’t working for you, and hopefully one of them will do the trick!