Image sources:

• – Caffeine:
  http://www.katom.com/Merchant2/graphics/clean2/blog/caffeine.jpg
• – Capsaicin:
  http://www.craphound.com/images/il_430xN.20102293.jpg
• – Morphine:
  http://graphics.ink19.com/issues/april2000/covers/morphine.jpeg
• – Nicotine:
  http://howlongdoesmarijuanastayinyoursystem.net/cigarette.jpg
• – Cannabinoids:
  http://itech.dickinson.edu/chemistry/wp-content/uploads/2008/04/thc-in-the-brain.jpg