Can I patent something that is used for an illegal purpose?

This question was posed to me during a recent interview. Is it possible to patent something that is used for an illegal purpose or is used with an illegal substance, or is there a prohibition against such patents? Is patent protection available for radar detectors, methods of cooking cocaine, marijuana pipes?

Generally, you can patent “anything under the sun that is made by man.” One of the requirements for patentability is that an invention be of the appropriate subject matter. Tangible inventions likely meet this requirement, but there are some limitations to protection, use, and ownership. For instance, atomic weapons have been carved out of patent protection – the Atomic Energy Act specifically eliminated this category of invention from patenting (though there is a large body of patents on devices used in atomic bombs, such as a switch that triggers on a change in air pressure). In other cases, patent applications relating to national security may be classified and held secret, but they can still issue as patents. And there is precedent for the government condemning a patent, or taking ownership in a patent through eminent domain when warranted by public interest.

There isn’t a prohibition on patenting something used for an illegal purpose or with an illegal substance, however. Moreover, such patents can likely be written broadly so that they encompass legitimate purposes or substances.

June 20, 2023 Update: Since this article was written, the courts and the Patent Office have drawn back on the “anything under the sung that is made by man” permissiveness of subject matter eligibility.  Many types of software inventions and business methods that were patentable back when this article was first written are now no longer patentable, and not because of whether they are new or not, but because the law has changed to exclude “abstract ideas” from the types of inventions that can even be patented.