The U.S. government spent 50 million dollars on research which showed that Truvada, an HIV medication, was safe and effective at preventing HIV transmission when taken by people who did not have the infection. As a result, the government received a patent on this application. However, they have never enforced their patent to collect royalties for its use.
Meanwhile, Gilead Pharmaceutical, the corporation that manufactures this drug, charges up to $2000 a month for this treatment. Since they started manufacturing this medication in 2004, they have collected 36.2 billion dollars in revenue from patients and their insurance companies.
Since the government owns the patent, public health experts are arguing that they can and should enforce the patent to require that Gilead lower the price of the drug (a one month supply of which is estimated to cost $6 to manufacture) to promote more widespread availability and reduce healthcare costs. Alternately, the government could collect royalties from the patent and use that money fund other HIV prevention and treatment efforts.
Either way, the government clearly has leverage to make HIV prevention more accessible and affordable and should absolutely use this power.