Gold has been used in the field of medicine for over four thousand years. The ancient Chinese used drugs containing gold to treat everything from smallpox and measles to joint and lung disease, apparently considering gold a sort of panacea. Medical manuals in seventeenth-century Europe suggested using gold for fevers, fainting, and melancholy. Even during the 1920s, gold therapy was a popular treatment for tuberculosis. Despite its popularity, however, the efficacy of medicinal gold was rarely supported by reliable scientific evidence. Because medicines and medical practice were not always well regulated, early doctors were free to prescribe treatments—such as gold—without proof that they actually worked.
What is the meaning of efficacy as used in the passage?