Administering or ingesting a combination of lysine with another agent for stress-related diseases other than lysine provides an improvement in the treatment and prevention of stress-related diseases (enhanced drug efficacy) and reduced side effects. Such combinations of lysine with another agent for stress-related diseases other than lysine can be employed in the form of a medicine (including a medicine for animals), a food, a drink or a feed, and are highly useful in preventing, treating, or ameliorating various stress-related diseases and preventing the progress of such diseases. The two ingredients as described above can be administered or ingested either at the same time or at different points and in different forms.