Pharmaceutical patents are not my area, and it's very specialized in that patent attorneys in working in the field tend to have related PhDs. Therefore, this answer will be evolving as I learn more. At a high-level, my understanding is that what is being patented are methods of production, delivery, or chemical composition of medicines, but absolute proof of efficacy seems not a requirement.
I'd highly recommend looking up some famous drug patents and reviewing the claims, and seeking out review articles on the subject from sites like ipwatchdog and The National Law Review, among others. Certain financial publications may also have relevant articles, such as this recent article on Forbes.
You may also want to look at USPTO guidelines for the US. It's fairly complicated.
- Utility of a drug or therapeutic technique is a requirement
An invention that is “inoperative” (i.e., it does not operate to produce the results claimed by the patent applicant) is not a “useful” invention in the meaning of the patent law ...
II.WHOLLY INOPERATIVE INVENTIONS; “INCREDIBLE” UTILITY | USPTO 2017 Guidelines for Examination
...the defense of non-utility cannot be sustained without proof of total incapacity.” If an invention is only partially successful in achieving a useful result, a rejection of the claimed invention as a whole based on a lack of utility is not appropriate.
Actual therapeutic efficacy seems not to be a requirement:
Courts have repeatedly found that the mere identification of a pharmacological activity of a compound that is relevant to an asserted pharmacological use provides an “immediate benefit to the public” and thus satisfies the utility requirement. As the Court of Customs and Patent Appeals held in Nelsonv.Bowler: "Knowledge of the pharmacological activity of any compound is obviously beneficial to the public."
It's a lot more complex than these two excerpts convey, but the cited source goes into much greater detail, with the relevant precedents.
- An examiner may request proof of utility
In appropriate situations the Office may require an applicant to substantiate an asserted utility for a claimed invention... (“When the operativeness of any process would be deemed unlikely by one of ordinary skill in the art, it is not improper for the examiner to call for evidence of operativeness.”)
ibid. | V.EVIDENTIARY REQUESTS BY AN EXAMINER TO SUPPORT AN ASSERTED UTILITY
Again, the cited document goes into much greater detail. You'll probably also want to review 2112 Requirements of Rejection Based on Inherency; Burden of Proof [R-07.2015]
I also found this interesting article that discusses patent protections for new uses of known drugs.