A manufacturer has a duty to warn consumers and others in the supply chain of all material risks of which it is aware, or of which it ought to be aware, resulting from ordinary use of its product.

The materiality of a risk depends on reasonable foreseeability (the likelihood of the risk materializing); the degree of harm should the risk materialize (ranging from inconsequential product malfunctions to severe health consequences); the obviousness of the risk (for example, the danger of cutting oneself with a knife would be the type of risk of which a manufacturer would not ordinarily be required to warn); and the audience to whom the warning would be given (for example, sophisticated professionals as opposed to children, etc.)

The duty to warn is a continuous one — that is, any time a manufacturer learns of a new risk, it is obligated to update the warnings it provides. The content of the warning must be adequate, understandable and clearly communicated, whether in labelling, instructions, or otherwise. The scope of the warning must be commensurate with the degree of risk or gravity of the hazard presented — the more serious the risk, the more obvious and easily understandable the warning.

As a general rule, where a product is only available through a “learned intermediary,” such as a doctor or authorized distributor, the detail and sophistication of the warning is expected to be more than that which would be given to the average consumer. For example, in the case of prescription drugs, a manufacturer is required to advise the prescribing physician of the risks of which it is aware, and to set out in detail all of the material scientific findings with respect to the product, so that the learned intermediary may evaluate whether use of the product is appropriate for a particular patient. The warning to the learned intermediary discharges the duty to warn the ultimate consumer and, except in very limited circumstances, there is no concurrent duty on a manufacturer to warn the ultimate consumer as it is expected that the intermediary will pass along the necessary information to the user.