“Plausibility” is not a ground for invalidity of a patent according to the European Patent Convention or the UK Patents Act. Nevertheless, it has become something of a buzzword in cases considered by the UK patent courts in recent years and it is important to understand its significance and to ensure that patent applications relate to a “plausible” invention from the outset.

A leading textbook for UK patent practitioners, Terrell on the Law of Patents, was first published in 1884. However, it was not until the 2016 edition that the concept of plausibility was included. Its inclusion has been in response to a significant increase in the number of cases in which the issue of plausibility has been considered during UK High Court proceedings, especially in life sciences cases.

The approach to plausibility has been relatively consistent before the UK Patents Courts. Plausibility is a “low, threshold test” and “it is not designed to prohibit patents for good faith predictions which have some, albeit manifestly incomplete, basis”. Furthermore, it has been emphasised that the test for plausibility is not aligned with the test for whether an invention would be obvious to try. This means that, for a claim to be plausible, the skilled person does not need to have a reasonable expectation of success, but rather merely a “reasonable prediction that the invention will work”. Nevertheless, plausibility can be a significant hurdle where the patent is fundamentally speculative in nature.

As a patent cannot be revoked for lacking plausibility, an allegation of a lack of plausibility must be linked to another deficiency of the patent that is a ground for invalidity.


In the UK, the disclosure of a patent may generally be considered to be insufficient for two reasons. First, a patent may lack sufficiency if it fails to describe the invention to the extent that it can be carried out by the  skilled reader without undue burden. Second, a patent may lack sufficiency if the claims are excessively broad so that the teaching of the patent does not allow the claimed invention to be performed across the whole scope of the claim.

Plausibility and sufficiency complement each other as they are both concerned with the extent to which the invention is disclosed in the patent.

In Generics (UK) Ltd (t/a Mylan) v Warner-Lambert Company LLC [2015] EWHC 2548, a claim directed to pregabilin for use in treating pain was found to be insufficient because there was no basis for concluding that it was plausible that pregabilin would be effective for treating all types of pain (although it was plausible that it could be used to treat certain specific types of pain). This decision appeared to set a reasonably high plausibility threshold but was upheld by the Court of Appeal.

In Actavis Group PTC EHF & Anor v Eli Lilly and Company [2015] EWHC 3294, Mr Justice Carr explained that the considerations underlying plausibility for sufficiency are different from those underlying fair expectation of success for obvious. Almost anything could be said to be obvious to try  and would lead to a denial of patent protection (and therefore research in pharmaceuticals) if it were not for the fair expectation of success requirement. Plausibility, on the other hand, is to exclude speculative patents, based on mere assertion, where there is no real reason to suppose that assertion is true.

Entitlement to priority

The relevance of plausibility to the question of entitlement to priority is very similar to its relevance to sufficiency. In order for a claim to be entitled to priority, the claimed subject matter must be sufficiently disclosed in the priority document. If the priority document fails to render the claimed subject matter plausible, the priority claim will be invalid.

Such an attack was used in Merck Sharp & Dohme v Ono Pharmaceutical [2015] EWHC 2973, which was concerned with the patentability of a claim directed to the use of a particular antibody in treating cancer in general. There was evidence indicating that the antibody in question was not successful in treating three particular types of cancer (colorectal cancer, prostate cancer and multiple myeloma).  Nevertheless, Birss J held that the mouse models for cancer in the priority document and the patent provided support for the claimed invention and rendered it plausible that the therapy would work to treat cancer in general, such that the patent was entitled to its priority date. According to this decision, a broad medical use claim was considered to be plausible even though the not all of the diseases claimed could be treated, although it is likely that this will depend on the specifics of the case, rather than being a general rule.


Plausibility is relevant in the consideration of obviousness in that only credible technical effects may be taken into account. In his judgment in Conor Medsystems Inc v Angiotech Pharmaceuticals Inc [2008] RPC 8, Lord Hoffmann set out that obviousness should be considered as a two step process, taking into account the plausibility of the invention. It should first be determined whether the claimed invention is plausible and then whether the claimed invention is obvious according to the usual UK approach (e.g. using the Windsurfer/ Pozzoli test).

This approach was adopted in Generics [UK] Ltd (t/a Mylan) v Yeda Research And Development Co Ltd and Teva Pharmaceutical Industries Ltd [2013] EWCA Civ 925. It was considered that a technical effect which is not rendered plausible by the patent specification may not be taken into account for assessing inventive step. This decision also confirmed that later evidence may be used to support a technical effect made plausible by the specification and that a  third party may rely on later filed evidence to challenge the existence of a technical effect made plausible by the specification.

Industrial applicability

In Human Genome Science v Eli Lilly and Company [2011] UKSC 51, the issue of plausibility arose when tackling the question of whether particular biological material had industrial applicability. Lord Neuberger set out that for a patent disclosing a new protein and its encoding gene, a plausible or reasonably credible claimed use or an educated guess of a use can suffice for demonstrating industrial applicability.


Plausibility can also be relevant to the assessment of novelty. It is well established that, for a disclosure to be novelty destroying, it must be enabling. The question of whether or not a disclosure is enabling is linked to its being sufficient. In Merck v Ono, the judge remarked that “plausibility is an aspect of enablement”. Thus, for the prior art to be an enabling, it must make the technical effect plausible. In the case at issue, the relevant prior art was not considered enabling because the document did not make it plausible that the particular agent would treat cancer.


It is clear from the decisions discussed above that failure to render the claimed invention plausible when drafting a patent  application can have serious repercussions, leaving the patent vulnerable to a number of different attacks and also undermining the prior art effect that the publication may have. That said, at present the threshold  for  plausibility is low and anything that indicates that the invention is not purely speculative in nature is likely to suffice to clear  the plausibility hurdle. However, any shift to raise this threshold could have significant consequences and so we will be watching the area with keen interest.

It is not hard to see how courts could use a stricter approach to plausibility as a means to clamp down on speculative patents benefiting patentees who have not really made a contribution to the art that justifies the monopoly they have been awarded.