A compound patent is valid for 20 years, and it will lose its protection after 20 years.
Using patents can be used to "continue" the monopoly position of patented technology.
Take aspirin for example,
It is the first time to use antipyretic and analgesic drugs.
Treating heart disease is a new secondary use.
New secondary utilization can still be patented.
In addition, if others also find a second use, they can apply for a patent. At that time, the company's compound patent expires and others hold the patent for use, so the production and sales will be controlled by others.