18511
Phosphodiesterase type 5 inhibitors ( PDE5 - Is ) improve cognitive performance of rodents , but the few human studies investigating their effects did not systematically investigate cognitive effects and the results have been quite contradictory .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None