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Bulk-forming laxatives

Key points about bulk-forming laxatives

Bulk-forming laxatives are used to prevent constipation.
They contain a type of fibre that absorbs water and swells to make your poo larger and softer.
Find out how to take them safely and possible side effects.

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Bulk-forming laxatives are used to prevent constipation. They contain fibre from plants that can’t be broken down by your gut. Instead, the fibre absorbs water, making your stool (faeces or poo) larger and softer.

The larger size of your stool activates the muscles in your gut to pass your poo and because the stool is softer, it also makes it easier to pass.

Bulk-forming laxatives aren’t helpful for the immediate relief of constipation. They can take 12 hours before you notice any changes and their full effect usually happens after a few days (1 to 3 days).

Eating a healthy diet, drinking plenty of water and getting regular gentle exercise can all help prevent constipation. Read more about constipation.

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Examples of bulk-forming laxatives

Natural forms of bulk-forming laxatives you can find in foods, such as cereals and biscuits, are unprocessed wheat bran or oat bran.

In Aotearoa New Zealand, bulk-forming laxatives are also available as powders and capsules. Some examples are Bonvit®, Konsyl-D® and Metamucil®. These are available to buy over the counter in supermarkets or pharmacies. You can also get them prescribed from your doctor or nurse prescriber.

Bulk-forming laxatives are best to use if you’ve already tried increasing fibre in your meals and you're still passing small, hard poos.

These medicines are helpful for conditions such as diverticular disease, irritable bowel syndrome, and haemorrhoids

Bulk-forming laxatives are not suitable if you:

are a child
have swallowing difficulties
are unable to drink enough fluids every day
are on a fluid-restricted diet
are taking some other medicines, especially lithium, clozapine or opioid medicines.

If you’re not sure if bulk-forming laxatives are suitable for you, ask your healthcare provider. Read more about the different types of laxatives.

Form: Bulk-forming laxatives are available as capsules or as a powder that you mix in water.
Dose: The dose of the different bulk-forming laxatives will be different, depending on the product.
Timing: Don't take bulk-forming laxatives immediately before going to bed.
Powder: Stir the correct amount of powder into a glass (250 mL) of water or juice for 3 to 5 seconds and drink straight away. Have a large glass of water afterwards.
Capsules: Swallow whole with a large glass of water.
Missed dose: If you forget to take your dose, take it as soon as you remember. But if it's nearly time for your next dose, just take the next dose at the usual time. Don't take double the dose.

Drink plenty of fluids

You need to drink plenty of fluids while you're taking a bulk-forming laxative – about 8 to 10 cups (2 litres) every day.

Not drinking enough fluid can cause a blockage in your gut, and make your constipation worse. Your poo 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.

Here are some things to know when you're taking bulk-forming laxatives. Other things may be important as well, so ask your healthcare provider what you should know about.

Other medicines: Avoid taking other medicine within 2 hours of taking bulk-forming laxatives. These medicines can interact with some medicines, herbal supplements and rongoā Māori, so check with your doctor or pharmacist before starting bulk-forming laxatives and before starting any new products.
Stay upright: Don’t lie down straight after taking 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. They may settle but tell your doctor if they get worse or these bother you.
Severe tummy pain and cramps Nausea (feeling sick) Vomiting (being sick)	Tell your doctor immediately or ring Healthline 0800 611 116.