Bulk-forming laxatives(ispaghula husk, methylcellulose and sterculia). These are recommended if you can't manage to increase the fibre in your diet. You need to drink at least 2 litres of fluid per day if you take these. They contain a type of fibre and work by increasing the amount of content in your bowel. As this builds up, it stimulates your bowel muscles to move and push the faeces out. It may take a few days for bulk-forming laxatives to work.
Attribution: BUPA