Given a situation, it's not realistic but need to figure out what's the economic concept behind it.

Assume that there exits a product(let's say some magical medicine) that can help people lose weight without putting any efforts in, but it can not be distributed to everyone who needs it at the same time. Suppose that there's enough for everyone but just a matter of time. While people who are waiting for the product, the government launch an advertisement campaign which tells people to do more exercise. Will the advertisement campaign be effective as people know they can get it later for sure and just need to wait. What is the potential problem called?

Thanks!