Final answer:
A patent owner can exclude others from making, using, or selling their invention, typically for a period of up to 20 years. Improving upon the invention is not prohibited unless it infringes the original patent.
Step-by-step explanation:
A patent owner has the exclusive legal right to control the use of their invention. This means they can exclude others from making, using, or selling the invention for a limited time. The length of a patent varies, but in the context given, a patent on a pharmaceutical drug can last for up to 20 years. After this period, the patent expires, and the invention enters the public domain, allowing others to make and sell it. It's important to note that while a patent holder can prevent others from making, using, or selling the invention, they cannot prevent others from improving upon the invention unless those improvements themselves infringe on the original patent or involve a new patentable invention.