The method you mentioned can be patented.
Maybe you don’t know much about patents.
During the patent review process, the effectiveness of the drug is not examined;
The review done by the examiner is based on the text submitted by the applicant.
In other words, the examiner conducts the examination on the basis that all the experimental data in the patent document are true.
Therefore, as long as the patent text meets the authorization conditions stipulated in the patent law (the technical solution is clear, complete, novel, creative, etc.), the patent authorization can be obtained.
The patent right itself is only a prohibition right, which can prohibit others from using the patent.
In other words, having a patent does not mean that the patentee can implement the content of the patent.
If the drug is really ineffective or harmful, other departments will review it and it will not harm the public interest.