caffeine. 164 Analogs Some analog substances have been created which mimic caffeine's properties with either function or structure or both. 138 Caffeine from coffee or other beverages is absorbed by the small intestine within 45 minutes of ingestion and distributed throughout all bodily tissues. High Altitude Medicine Biology. "Is caffeine a cognitive enhancer?". Muriel P, Arauz J (July 2010). Funk GD (November 2009). The pyrimidinedione in turn contains two amide functional groups that exist predominately in a zwitterionic resonance the location from which the nitrogen atoms are double bonded to their adjacent amide carbons atoms. Have sleep disorders, including insomnia, have migraines or other chronic headaches, have anxiety Have gerd or ulcers Have fast or irregular heart rhythms Have high blood pressure Take certain medicines or supplements, including stimulants, certain antibiotics, asthma medicines, and heart medicines.