Product liability law protects consumers against harms suffered due to defects in the products they purchase. The seller of a defective product can be held liable for the consequences of selling defective merchandise. However, products can pose dangers to users even when there are no defects in the manufacturing process and the products are not designed defectively. For example, infant car seats can be very dangerous and lead to serious harm if they are used improperly even if they are well designed and are manufactured according to specifications.
To address such situations, the law imposes a duty on manufacturers and sellers of products to give adequate warnings of the dangers that can arise from foreseeable uses of their products. Those who fail to adequately warn users can be liable for damages stemming from the failure to warn.  It is important to emphasize that an effective warning precludes liability only when a product is not otherwise defective. A warning will not absolve a manufacturer from liability if the product in question has a manufacturing or design defect.
The law considers an inadequate warning to be equivalent to providing no warning at all. The standard for what constitutes an adequate warning is very broad. It is what a reasonably prudent manufacturer would provide as a warning under similar circumstances. Generally, adequate warnings will include a description of the nature of the hazard, clear instructions for avoiding the hazard, as well as the potential consequences of failing to adhere to the warnings.
However, the law does not require extensive detail regarding all the possible harmful consequences or minute specifics of how harm can occur. In fact, excessive detail may impede the user from understanding the most important elements of the danger, thus defeating the purpose of the warning. The warning must also be expressed in a manner that makes it apparent to the user. So, it must be in a form that would catch the eye of a reasonably observant person and should also be comprehensible to the average user. The use of small print, or the placement of the warning in a location where the user is unlikely to notice, can render the warning legally inadequate.
When is a Warning Required
A product will be considered defective if a manufacturer failed to warn users of foreseeable risks posed by the product and those risks could be reduced or avoided by a clear warning or set of instructions. The duty to warn is applicable if three conditions are met. First, the manufacturer knew or should have known that the product poses substantial risk to the user; second, the danger would not be obvious to users; and third, that the risk of harm justifies the cost of providing a warning.
To satisfy the first element, knowledge of a substantial risk, the plaintiff must show that the defendant knew or reasonably should have known of potential dangers. Whether a manufacturer should have known of likely harm from foreseeable misuse depends on the state of knowledge of the scientific and manufacturing communities. A manufacturer is expected to have the knowledge that an expert in the field of the manufacturer’s specialty would have at the time the product was offered for sale. 
Conversely, this means the defendant can argue against a finding of liability based on the lack of general awareness of the dangers associated with the product in the scientific community. This defense was successful, for example, in defeating a claim of liability based on exposure to asbestos. The court ruled that a manufacturer could not be held liable for failing to warn of dangers which were not known in the medical and scientific fields when the plaintiff purchased the product.
However, a manufacturer cannot escape liability by neglecting to take steps on its own to discover knowable dangers. A seller is considered responsible to know what reasonable testing of the product for dangers would reveal. This, in effect, creates an obligation on manufacturers to test products, in addition to the obligation to warn users of known dangers.
Manufacturers also have a duty to warn against misuses of their products when such misuses are reasonably foreseeable. Therefore, instructions for proper use should consider the fact that users of products may use them differently from the way intended by the manufacturer. Cases in which courts have deemed misuses as reasonably foreseeable include using toy weapons to cause injury, applying heating devices to the skin without insulation, and the removal of safety features from dangerous machines. In circumstances such as these, manufacturers must warn users of the dangers posed by the products, even though they were used incorrectly.
When the danger presented by a product is open and obvious, there is no need for a warning. The obviousness of the danger for the average user serves as a sufficient warning. This is the reason that common household implements like knives do not need warnings. The dangers arising from ordinary use are familiar to everyone.
Nevertheless, there is a tendency for courts to require warnings even when a danger is obvious to many. This is largely because of the third factor in determining when a warning is necessary, which weighs the cost of a warning against the risk of harm.
Since the cost of including a warning with a product is usually minimal, there may be a duty to warn of seemingly obvious dangers if there is likely to be some significant group of people for whom the danger is not obvious. In addition, even when a danger is obvious, there is value in advising users that there is a safer way to use these dangerous products.
This was the reasoning of the court in a case involving a teenage store employee who was severely injured by a meat grinder. He used the grinder after the safety guards had been removed by the store supervisor. The judge in the case conceded that the danger of using the meat grinder without the safety guards was obvious. However, he held that it was reasonable to conclude that a substantial number of people would not have known there was an alternative to using the machine without its safety features. Therefore, the manufacturer was required to include a warning stating that the machine should not be used without its safety guards.
The Requirement of Causation
As with other claims for personal injury, the plaintiff must demonstrate that the defendant’s conduct caused his or her injury. When it comes to the duty to warn, the conduct in question is the lack of an adequate warning. So, some courts have required plaintiffs to specifically demonstrate that the lack of warning caused the accident. They must show that the harm would not have occurred but for the failure to warn.
In these jurisdictions, a defendant can argue that the plaintiff failed to offer evidence demonstrating that he or she would have heeded an adequate warning if one had been provided. If no such evidence has been offered, the plaintiff has not shown that the lack of a warning caused the injury.
One such example involves a pedestrian who was hit by a truck that appeared to have the engine turned off. When the plaintiff approached, the truck suddenly lurched forward and struck the plaintiff, causing severe injury. The court ruled against the plaintiff on the grounds that he failed to show that he would have heeded a warning that instructed nearby pedestrians not to approach the truck.
In other jurisdictions, however, if an adequate warning is provided, it is legally presumed that it would be heeded. The burden is on the defendant to prove that the victim of the harm would not have heeded an adequate warning if one were provided.
Post-Sale Duty to Warn
While the duty to warn is initially limited to dangers known at the time of sale, the law recognizes that knowledge about the quality and safety of products is constantly being updated. Therefore, many jurisdictions have extended the obligation to warn of risks that have been discovered after the product has been sold.
The conditions that create a post-sale obligation to warn are like those that require warnings at time of sale. A post-sale warning obligation comes into effect when a merchant knows or should know of a substantial risk posed by a product, and the degree of risk justifies the cost of providing a warning. Typically, these risks are discovered through widespread use of the product once it is on the market. When it can be verified that such dangers exist, manufacturers must not only redesign the product or warn new purchasers, but also issue a warning that reaches existing users of the product.
However, issuing an effective warning after customers have already purchased products and taken them home often involves logistical problems. So, whether there is a post-sale duty to warn also depends on practical factors, such as whether those who would benefit from a warning can be identified and if the warning can be effectively communicated. A defendant would be not be liable for injuries if it can be shown that issuing a warning to the plaintiff involved substantial practical difficulties.
These difficulties were sufficient for a court to reject liability in a case involving a man who lost four fingers using a snow blower. While the manufacturer did learn of specific risks after the product reached the market, the plaintiff had purchased the snow blower second hand, and did so sixteen years after the initial sale. He was deemed too remote from the initial marketing of the product and the discovery of the dangers to impose a duty on the manufacturer to contact him with a warning.
Finally, some courts have extended the post-sale obligation to warn to a general duty to monitor and test the performance and safety of products after sale. This is especially true in cases involving prescription drugs which pose a distinct set of risks to the consumer. Note that while many manufacturers will issue product recalls when learning of substantial defects, there is generally no liability for failing to recall products, unless an obligation to recall is set by statute.
Special Cases: Prescription Drugs and Cigarettes
The distribution of prescription drugs represents a special case of the duty to warn of a product’s latent risks.
Generally, prescription drugs are not available to the public without the authorization of a physician. In addition, the nature of the dangers of using and misusing prescription drugs is highly complex, and often not readily understood by the average user. These factors mean that issuing a warning to the end user of the drugs may not be an effective way of preventing harm. The law recognizes that physicians are more knowledgeable about the benefits and risks of using medical drugs for individual patients, and so are in a better position to decide whether to prescribe specific drugs and to explain the risks to his or her patients.
So, unlike for non-medical products, drug manufacturers have a duty to warn the physicians or medical personnel who prescribe the drugs, rather than the patient who will use them. This is known as the learned intermediary doctrine.
However, if the manufacturer knows that the prescribing doctor or healthcare provider is not in a position to issue the warning to the users, the manufacturer must directly warn the patient. Two important examples of circumstances under which a manufacturer must warn the user directly are birth control pills and mass inoculations. Courts have held that users of these drugs rarely rely on consultations with physicians, and so have mandated that the drug companies include warnings designed for the users themselves.
Another special case for duty to warn is the case of cigarettes.
Due to the dangers of smoking and the history of deceptive advertising, the sale of cigarettes is highly regulated. Ever since the Cigarette Labeling and Advertising Act of 1965, it has been illegal to distribute or sell cigarettes without a conspicuous warning on the box stating that smoking cigarettes may be hazardous to the smoker’s health. This legislation has had an impact on the scope of civil litigation for failure to warn when it comes to cigarette distributors.
The US Supreme Court ruled that the federal act preempts all other law governing the same area. This means that since the federal act mandated a specific warning for cigarettes, this requirement takes precedence over any other more demanding warning requirement for cigarettes at the state level. Therefore, cigarette manufacturers who comply with the federal law cannot be held liable for failing to issue an adequate warning.
In the next module, we will turn to the scenario where the product is not necessarily defective, but where it fails to live up to the promises (express or implied) made by the manufacturer or seller.
 See Davis v. Wyeth Laboratories, 399 F.2d 121 (9th Cir. 1968).
 Restatement (3rd) of Torts: Prods Liab., §2, comment l.
 American Law of Products Liability 3d, §33:1.
 Gurley v. Honda, supra, 505 So.2d at 361.
 Shanks v.Upjohn Co., 835 P.2d 1189, 1200 (Alaska 1992).
 Hood v. Ryobi America Corp., 181 F.3d 608 (4th Cir. 1999).
 Restatement (3rd) of Torts: Prods Liab., §2, comment i.
 BituminousCasualty Corp. v. Black & Decker Manufacturing Co., 518 S.W.2d 868 (Tex.App. 1974).
 See McLaughlin v. Mine Safety Appliances Co., 11 N.Y.2d 62, 181 N.E.2d 430, 226 N.Y.S.2d 407 (1962).
 Restatement (3rd) of Torts: Prods Liab., §2(c).
 Anderson v. Owens-Corning Fiberglas Corp., 810 P.2d 549, 53 Cal. 3d 987, 281 Cal. Rptr. 528 (1991).
 Restatement (3rd) of Torts: Prods Liab., §2, comment m.
 Lugo v LJN Toys, 75 N.Y.2d 850.
 Lugo v LJN Toys, 75 N.Y.2d 850; Liriano v. Hobart Corp., 700 N.E.2d 303, 92 N.Y.2d 232, 677 N.Y.S.2d 764 (1998); McLaughlin v. Mine Safety Appliances Co., 11 N.Y.2d 62, 181 N.E.2d 430, 226 N.Y.S.2d 407 (1962).
 Restatement (3rd) of Torts: Prods Liab., §2, comment j.
 See Liriano v. Hobart Corp., 700 N.E.2d 303, 92 N.Y.2d 232, 677 N.Y.S.2d 764 (1998).
 Liriano v. Hobart Corp., 700 N.E.2d 303, 92 N.Y.2d 232, 677 N.Y.S.2d 764 (1998).
 Corbo v. TAYLOR-DUNN MANUFACTURING COMPANY, No. A135393 (Cal. Ct. App. Feb. 14, 2014).
House v.Armour of Am., Inc., 886 P.2d 542, 552-53 (Utah Ct. App. 1994).
 Restatement (3rd) of Torts: Prods Liab., §10(a)-(b).
 See Comstock v. General Motors Corp., 99 N.W.2d 627, 358 Mich. 163, 358 Michigan 163 (1959).
 Restatement (3rd) of Torts: Prods Liab., §10(b).
 Restatement (3rd) of Torts: Prods Liab., §10, Illustr. 1.
 Lewis v. ARIENS COMPANY, 434 Mass. 643, 751 N.E.2d 862 (2001).
 Restatement (3rd) of Torts: Prods Liab., §10, comment c.
 Restatement (3rd) of Torts: Prods Liab., §10(b).
 Restatement (3rd) of Torts: Prods Liab., §6(d)(1).
 Restatement (3rd) of Torts: Prods Liab., §6(d)(2).
 See MacDonald v. Ortho Pharmaceutical Corp., 475 N.E.2d 65, 394 Mass. 131 (1985).
 15 U.S.C. ch. 36 § 1331 et seq.
 Cipollone v. Liggett Group, Inc., 505 U.S. 504, 112 S. Ct. 2608, 120 L. Ed. 2d 407 (1992).