What is a failure to warn claim against a manufacturer in Ontario?

A failure to warn claim argues that a product itself may have been reasonably designed and manufactured, but that the manufacturer did not adequately warn users about a known, or reasonably foreseeable, risk associated with using it, and that inadequate warning is what caused the injury - not a flaw in how the product was built. Manufacturers generally have an ongoing duty to warn about risks known at the time a product is sold and, in some circumstances, about serious risks that become apparent afterward, particularly where the danger involved is not obvious to an ordinary user just from looking at or using the product.

Whether a specific warning was adequate is judged on more than whether one existed at all; courts look at whether it effectively conveyed the nature and seriousness of the risk to the people actually expected to use the product, in a way they would reasonably notice and understand. This is a distinct legal theory from a design or manufacturing defect claim, and it can succeed even where a product otherwise performed exactly as it was intended to, if the real problem was that users were never properly told what they needed to know to use it safely.