Sulforaphane
Sulforaphane side effects
What trials have reported, and the groups with a real reason to be careful.
Quick answer
In published trials, sulforaphane and broccoli sprout preparations have most often been tolerated without serious adverse effects, with digestive complaints such as gas and bloating the most commonly reported problem. The trials are mostly small and short, so they cannot rule out uncommon or long-term effects.
What the trials report
Most human studies of sulforaphane have been small, running from a few dozen to a few hundred participants over weeks rather than years. Within those limits, the commonly reported complaints are digestive: gas, bloating and loose stools. Some participants find the taste of sprout preparations difficult, which shows up in trials as poor compliance rather than as an adverse event.
The honest limitation is that a trial of forty people for eight weeks cannot detect an effect that appears in one person in a thousand, or after two years. Absence of reported harm at that scale is weak evidence of safety, not strong evidence.
Who has genuine reason for caution
Cruciferous vegetables contain compounds that can interfere with thyroid iodine uptake, which is relevant at supplement doses rather than normal food amounts. Anyone being treated for a thyroid condition should raise it with their doctor before taking a concentrated product.
Isothiocyanates have been reported to affect platelet function, so anyone on anticoagulant or antiplatelet medication should do the same. There is not enough data on pregnancy or breastfeeding at supplement doses to say anything useful, which is itself a reason for caution.
None of this applies to eating broccoli.
Common questions
Can you take sulforaphane every day?
Does sulforaphane affect the thyroid?
Why do broccoli sprout supplements cause gas?
This page describes published research. It is general information, not medical advice, and it is not a recommendation to take anything. Sulforaphane and cruciferous-vegetable supplements can interact with medication. Talk to a doctor or pharmacist before starting a supplement, particularly if you are pregnant, breastfeeding, on anticoagulants, or being treated for thyroid disease.