C8H10N4O2. Caffeine is a stimulant. Caffeine can cause anxiety, nervousness, headaches, tremors, nausea, and heart palpitations. If you overuse caffeine, you may be stricken with dizziness, and flushed skin. The average cup of brewed coffee has approximately 135 milligrams of caffeine. It would take less than 100 cups of finely brewed, aromatic coffee to kill me. With constant ingestion of caffeine you can develop a tolerance- your own body's natural defense against the constant and elevated levels of the stimulant coursing through your body.
CJH. Captain Jack Harkness is a stimulant. He can, with something as simple as a look, cause me great anxiety, and nervousness. With a look, or suggestive, whispered words he can inflict me with tremors and heart palpitations. His scent, and the touch of his skin are infectious; intoxicating, and merely thinking about him make me dizzy and causes my skin to flush. It is impossible to build up a tolerance or defenses to his charm and charisma. The withdrawal of his affections leaves me unable to eat, unable to breath, nauseated. It will only take one Jack Harkness to kill me.