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The phosphodiesterase-5 inhibitor study was performed independently along with the flow chart by two reviewers: (1) Duplicate articles were excluded. (2) Publications with unrelated titles and abstracts were excluded. (3) According to the eligibility, publications that had no therapeutic effect in the behavioral experiment were excluded. (4) Full-text articles that were not available for meta-analysis, that had no animal, cognition, or Morris water maze test, were excluded.
