Bulk-forming laxatives

Also called fibre supplements

Bulk-forming laxatives are used to prevent constipation. They are also called fibre supplements. Find out how to take it safely and possible side effects.

Examples of bulk-forming laxatives
  • Bonvit®
  • Konsyl-D®
  • Metamucil®
  • Mucilax regular®
  • Normacol®
  • Unprocessed wheat bran
  • Oat bran

What are bulk-forming laxatives?

Bulk-forming laxatives are used to prevent constipation. They are also called fibre supplements.

  • Fibre is the part of plants that is not digested and stays in your gut and is passed in the stool. 
  • Bulk-forming laxatives add bulk to the stool (faeces) by absorbing water into the gut, making the stool softer and easier to pass.
  • Bulk-forming laxatives also swell or expand in the gut and activate the muscles in the gut to pass stools (faeces). 

Read more about constipation.

Bulk-forming laxatives do not work immediately

Bulk-forming laxatives usually take a few days (1 to 3 days) to work. They are not helpful for the immediate relief of constipation. They are best to use if you want to have bowel motions more often and are helpful for conditions such as diverticular disease, irritable bowel syndrome, and haemorrhoids.

Bulk-forming laxatives are not suitable if you have swallowing difficulties or are unable to drink enough fluids every day.

If you are unsure if bulk-forming laxatives are suitable for you, ask your doctor or pharmacist. Read more about the types of laxatives.

How to take bulk-forming laxatives?

  • The dose of the different bulk-forming laxatives will be different, depending on the product. 
  • Bulk-forming laxatives are available as capsules, granules or as a powder that you mix in water.
  • Follow the directions on the label or the packaging. It will tell you how much to take, how often to take it, and any special instructions.
  • Do not take immediately before going to bed.
  • Avoid taking other medication within 2 hours of taking bulk-forming laxatives.

Drink plenty of fluids

You need to drink plenty of fluids while you are taking a bulk-forming laxative – about 8 to 10 cups (2 litres) every day. Not drinking enough fluid can cause blockage in the gut, and make your constipation worse. Your poos will be dry and difficult to pass. Take each dose in or with a full glass or more of water. This will provide enough liquid for the laxative to work properly. Continue to drink enough water throughout the day.

What are the side effects of bulk-forming laxatives?

Like all medicines, bulk-forming laxatives can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.

Side effects What should I do?
  • Bloating
  • Gas in the stomach
  • Cramps
  • These are quite common when you first starting taking a bulk-forming laxative
  • This may settle but tell your doctor if troublesome or gets worse
  • Severe tummy pain and cramps, nausea and vomiting
  • Tell your doctor immediately or ring HealthLine 0800 611 116 
Did you know that you can report a side effect to a medicine to CARM (Centre for Adverse Reactions Monitoring)? Report a side effect to a product


  1. Laxatives New Zealand Formulary

Additional resources for healthcare professionals

Irritable bowel syndrome in adults: Not just a gut feeling BPAC, NZ, 2014

Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 25 Sep 2018