Reader Question: Why no garlic before colonoscopy?

"I saw garlic on a ‘don’t eat’ list before a colonoscopy — does anyone know why that is? Seems kind of random. Some sources don’t mention garlic at all, so is it really a problem?"

I understand your scepticism, but I assure you that sources that list garlic as part of foods to avoid are not doing so randomly. Garlic sometimes shows up on colonoscopy “avoid” lists because of two main reasons:

1. Fiber and residue – While garlic cloves are small, they do contain fiber and small particles that can linger in the digestive tract. If eaten in larger amounts (like roasted cloves or garlic-heavy dishes), those bits can make it harder to fully clear the colon, which may reduce visibility during the exam.

2. Blood-thinning effect – Garlic, especially in supplement form or when eaten in large quantities, can have mild blood-thinning properties. If your doctor plans to remove polyps or take biopsies, they generally prefer you avoid foods or supplements that might slightly increase bleeding risk.

Occasional small amounts of garlic used in cooking usually aren’t a problem - unless your doctor’s prep instructions specifically say otherwise. The stricter “no garlic” advice is often aimed at patients who take garlic supplements, eat raw garlic in significant quantities, eat very garlicky foods regularly, or are on other blood-thinning medications.

In conclusion, your prep sheet says “no garlic,” so follow it to the letter. Colonoscopy prep works best when instructions are followed precisely. If it’s not mentioned, small amounts in cooked meals are usually acceptable, but skip garlic supplements or heavy raw garlic dishes in the days before your procedure.

Sources:

https://www.endoscopyclinic.co.nz/wp-content/uploads/2022/04/Medication-considerations.pdf