Failure to warn is a distinct branch of products liability from manufacturing and design defect claims: the product can be built exactly as intended and designed as safely as feasible, and still give rise to liability if the manufacturer failed to adequately warn users of a danger the product presents that is not obvious and that the manufacturer knew or should have known about. The defect is in the information that accompanied the product, not in the product itself.

Because the claim does not require showing a safer design was feasible, it is often pursued alongside or instead of a design-defect theory, particularly for products -- pharmaceuticals, chemicals, machinery -- where some risk is inherent and unavoidable even in a well-designed product, and the only real question is whether users were adequately told about it.

A plaintiff generally must show: (1) the product had a danger associated with its use or foreseeable misuse; (2) the manufacturer or seller knew or, in the exercise of reasonable care or through application of reasonably developed human skill and foresight, should have known of the danger; (3) the danger was not obvious or generally known to the ordinary user; (4) no warning was given, or the warning given was inadequate; and (5) the inadequate or absent warning caused the plaintiff's injury.

The 'known or should have known' standard generally ties the manufacturer's duty to the state of scientific and technical knowledge available at the time the product was sold or distributed, not to hindsight knowledge developed later -- an important limit that keeps the claim closer to a negligence-based standard even in strict-liability jurisdictions.

An adequate warning must be reasonably calculated to reach the persons likely to be endangered, must communicate the nature and extent of the danger with sufficient specificity and force to be understood, and must be conspicuous enough -- in placement, size, and signal words -- that an ordinary user would actually notice it. A warning that is technically present but buried, vague, or understated can still support liability as inadequate.

Courts also apply the open and obvious danger doctrine as a limit: manufacturers generally have no duty to warn of dangers that are patently obvious or already generally known to the ordinary user, since a warning adds nothing when the risk is already apparent. Where an intermediary -- a treating physician for a prescription drug, an employer for workplace equipment -- learns the danger and has an independent duty to pass it on, some jurisdictions apply a 'learned intermediary' rule treating an adequate warning to the intermediary as satisfying the manufacturer's duty to the ultimate user.

Plaintiffs build the claim around the actual warning given (or its absence), expert testimony on what the manufacturer knew or should have known at the time of sale, and evidence the danger was not obvious to an ordinary user -- along with causation evidence that a better warning would actually have changed the plaintiff's or an intermediary's behavior. The causation element is often the hardest to prove: if the plaintiff would have used the product the same way regardless of any warning (the 'heeding presumption' cuts both ways depending on jurisdiction), the claim can fail even with a genuinely inadequate warning.

Defendants attack by showing the danger was open and obvious, that the warning actually given was adequate as a matter of law, that the risk was not knowable given the scientific state of the art at the time of sale, or that a learned intermediary's independent knowledge and judgment broke the causal chain to the ultimate user.

In Juricratic terms, failure to warn decomposes into a knowability dial (state-of-the-art evidence of what was knowable when sold), an obviousness gate (whether the danger was open and obvious to an ordinary user), an adequacy dial for the warning actually given, and a causation dial -- sharpened by a learned-intermediary gate in relevant industries -- connecting the inadequate warning to the plaintiff's actual injury.

Because failure to warn frequently rides alongside a design-defect claim on the same product, the model can run both theories in parallel and show where the warning theory carries exposure even if the design-defect theory fails on feasibility grounds. These are simulation dials, not a claimed prediction of how a specific jury weighs warning adequacy.