Caffeine is physically and psychologically addicting. Within the first day of quitting caffeine symptoms will include a lack of alertness, you will be mentally foggy, your muscles will be tired, and you will be more irritable than normal. Over time, you will get a bad headache and feel small muscle aches, nausea, and other flu-like symptoms.
In, 1995 NASA tested the effect of multiple drugs on spiders and their ability to spin webs. Spiders can't metabolize as well as humans, meaning they were much more affected by the drug.