4000
Participants were then tested on 2 occasions after 12 - hour abstinence from caffeine in each of 2 protocols ; this required a total of 4 laboratory visits .
 Chemical: caffeine
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 Chemical: caffeine