Sildenafil is the active ingredient in Viagra, the blockbuster potency-enhancing drug marketed by Pfizer. The Canadian sildenafil patent saga came to an end on November 8, 2012, when the Supreme Court of Canada declared the patent void, a year and a half prior to its expiry. In doing so, the Supreme Court clarified the law on sufficient disclosure, as well as the relationship between a patent, the invention it protects, and the claims which define the patent monopoly. In its judgment, the Supreme Court also provided a few tantalizing hints about its views on disclosure and the doctrine of sound prediction, but declined to rule on that issue, as the case involved demonstrated, rather than predicted, utility.

The facts in the Pfizer decision are uncharacteristically straightforward. The patent claims were constructed using a cascading structure of claims, with the broadest independent claim, Claim 1 disclosing "formula I" – which included 260 quintillion possible compounds. This massive family of compounds was then progressively narrowed in dependant Claims 2 through 5, with Claim 5 consisting of only nine compounds. Of these nine compounds, Claims 6 and 7, described in the patent as "especially preferred embodiments" of the invention, each disclosed a single and distinct compound. Sildenafil was the compound disclosed in Claim 7.

Since sildenafil already existed at the time the patent was awarded, the inventive concept of the patent was not sildenafil itself, but rather the use of sildenafil for the treatment of erectile dysfunction. Teva argued that since the claims of the patent, as drafted, ended with two individually claimed compounds, only one of which was sildenafil, the true invention was obscured.  As such, the disclosure failed to state the invention in clear terms.

Pfizer denied the ambiguity of the patent and added that in light of case law and section 58 of the Patent Act, courts are required to consider valid and invalid claims separately. Thus, even if the patent's Claims 1 to 5 were held invalid, the invalidity of those claims would not affect Claim 7 thereby ensuring that sildenafil would remain protected by the patent monopoly.

Pfizer's arguments regarding the severability of claims succeeded in the lower courts. Despite the Federal Court's characterization of Claims 1 to 5 as "red herrings" creating ambiguity in Pfizer's patent, the patent was upheld.  Guided by existing case law, the Court examined the validity of each claim within the patent on an individual basis. Although the trial judge doubted the validity of Claims 1 to 5, when he arrived at Claim 7, he concluded that it was valid since it disclosed a single compound. As such, there could be no ambiguity and Teva's request for authorization to produce a generic version of Viagra was denied.

The Federal Court of Appeal also upheld the validity of the patent on the basis of its construction of section 58 of the Patent Act. The earlier claims were problematic because they disclosed a class of compounds without providing the reader with a mechanism to select the sole effective compound from the class. But Claim 7 disclosed a single compound thereby obviating the selection problem. The Federal Court of Appeal further held that the sufficiency of the patent's disclosure could be assessed in answering two questions: "What is the invention?" and "How does it work?" The Court of Appeal found that both requirements were met by Claim 7.

In a unanimous (7-0) decision, the Supreme Court of Canada overruled the lower courts on two grounds. First, the Supreme Court held that a patent must be examined as a whole, rejecting the claim-by-claim analysis used by the lower courts. The Supreme Court noted that while section 58 of the Patent Act provides that valid claims are not affected by invalid claims, this provision does not mandate that courts assess a patent's validity on a claim-by-claim basis. Instead, the Supreme Court explained that a patent discloses a single invention, and that it is this invention which must satisfy the validity requirements of the Patent Act. The patent's claims merely describe the scope of the economic monopoly. Thus, if the patent as a whole fails to sufficiently disclose the invention, the patent cannot be saved by inviting the judge to apply the sufficient disclosure test to a single claim, as had occurred in the lower courts.

Second, the Supreme Court held that the lower courts had misunderstood the test for sufficient disclosure. The Supreme Court clarified that the relevant test was not the simple two-question test used by the lower courts (i.e. "What is your invention? How does it work?"). Rather, the correct test was whether a skilled person could, using only the information contained in the specification, produce the invention described therein. Pfizer's patent required of users to perform experiments in order to discover whether it was Claim 6 or Claim 7 that disclosed the effective compound. Since the patent required further experimentation in order to put the invention into practice, there was insufficient disclosure. As such, the patent failed to enable the public to decipher that sildenafil, as opposed to the other compounds claimed within the class, was the active and useful compounds for the treatment of erectile dysfunction.  Consequently, the patent was void