Is fiber safe?

Side effects in trials

A 2016 Cochrane review of 23 trials found mostly mild to moderate gut side effects, and they were more common with fiber than in control groups.^[1] A 2022 review of 16 trials in constipation found more gas with fiber than with control treatment.^[2]

Eating a lot of fiber in a short time can cause gas, bloating and cramps. This often goes away when the gut bacteria get used to the increase.^[5] A review for clinicians gives a slow start: add 2 to 3 g every few days, and drink enough water.^[6] Gas from psyllium generally disappears during treatment, according to the European Medicines Agency (EMA).^[3]

Choking and a blocked gut

Psyllium swells when it meets liquid. With too little liquid, bulk fibers can block the throat, the esophagus (the tube from the mouth to the stomach) or the gut.^[3] The EMA says at least 30 ml of liquid per 1 g, and MedlinePlus says at least 240 ml per dose.^[3][4]

Health Canada says to contact a health professional at once after chest pain, vomiting, or trouble swallowing or breathing after a dose.^[7]

The EMA says psyllium husk should not be used by people with:^[3]

• trouble swallowing or throat problems^[3]
• a narrowed gut, a blocked gut or a risk of one^[3]
• a sudden change in bowel habits that lasts more than 2 weeks^[3]
• bleeding from the rectum with no known cause^[3]

Allergy

Psyllium contains strong allergens. Reactions include a runny nose, inflamed eyes, narrowed airways, skin rash and, in some cases, anaphylaxis, a severe allergic reaction.^[3] The risk is higher for people who breathe in the powder often, such as health workers and caregivers.^[3]

Medicines and minerals

Psyllium can delay the absorption of other medicines, vitamin B12 and minerals.^[3]

Too much fiber may reduce the absorption of iron, zinc, magnesium and calcium. MedlinePlus says this is usually not a large concern, because high-fiber foods tend to be rich in minerals.^[5] In a small study of 15 women, 3.4 g of psyllium made no significant change in the absorption of calcium from a drink.^[8]

How long it can be taken

Sources differ. MedlinePlus says not to take psyllium for more than 1 week unless a doctor says so.^[4] A trial for high cholesterol gave it for up to 6 months, in general practices and hospital centres.^[9]

Pregnancy and children

The EMA reports limited data, from fewer than 300 pregnancies. It says psyllium husk may be considered in pregnancy and breastfeeding if needed and if a change in diet has not helped.^[3]

The EMA does not recommend psyllium husk under age 6 for constipation or under age 12 for other uses, because of too little data.^[3]

Who should ask first

People with diabetes or people who take thyroid hormone should use psyllium only under medical supervision, because their medicine dose may need to change. The same applies to frail or older people.^[3] Anyone who takes a medicine every day should ask a doctor or pharmacist first.^[3][4]