Sulforaphane side 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 compete with iodine for uptake by the thyroid. A review of the human data found that ordinary cruciferous intake does not raise hypothyroidism risk where iodine intake is adequate, so the question is concentrated supplement doses rather than food. Anyone being treated for a thyroid condition, or with low iodine intake, should raise it with their doctor before taking a concentrated product.

Sulforaphane reduces platelet clumping in isolated human platelets in the laboratory, and has an antithrombotic effect in animal studies. No human trial has tested whether it changes bleeding risk, so the practical significance is unknown. Anyone taking an anticoagulant or antiplatelet medicine should still raise a concentrated supplement with their doctor. 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.