11149
Following repetitive administration ( 2 . 5 mg / kg / day during 10 days ) , we observed a 13 - fold increase in the effective dose - 50 % ( ED 5 0 ) of morphine - induced analgesia in comparison to a 2 - or 4 - fold increase in the ED 5 0 of its related increase in inspiratory time determined in air and 4 % CO 2 , respectively .
 Chemical: morphine; Chemical: CO 2
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: morphine; Chemical: CO 2