season 7 scrapped the drug analogy and as for Willow's withdrawl symptoms in season 6 they can be explained a couple of ways. We don't know what spells Rack used. In season 1 Catherine Madison used a spell that initally made Buffy act drunk then made her very sick.
And of course Willow was constantly using a lot of magic. In season 5 when she used a lot or did powerful spells she got a nosebleed one time and headaches others. I don't know if it was ever officially called burnout, but I recall seeing the term burnout. What happened in season 6 when she was supposedly detoxing could have been a hyper advanced form of burnout.