Boric acid is a common ingredient found in many different solutions. Its uses vary enough even to be employed against infections in humans. It is an effective killer of insects, but its effects against rats are less substantiated.
Boric acid is frequently marketed as a great extermination tool for cockroaches, fleas and other insects. Smaller concentrations of it may be used and will be effective, but it is not commonly recommended for killing larger pests, like rats and snakes.
Studies conducted by California health researchers in the 1980s tested the effects of boric acid on mice and rats in a lab environment. It was shown to consistently stop the animals' reproductive functioning and development, but there is no mention of it being an aggressive or effective extermination method for rats.
Killing rats by poison has one serious drawback. The animal may crawl into an unreachable place before it dies, leaving it to decompose there. If it is indeed carrying disease, or simply due to the odor, this is not desirable. It's best to use a trap that will allow you to properly dispose of it.