I've been wondering this myself. Does somebody who takes the goblin formula have to continue taking it to reap the effects, or does it mutate their DNA after one go? Norman seems to still possess his super-strength and intellect from his original exposure to the serum, but since his return Harry has shown no sign of having superhuman powers at all. Plus, it was, in fact, the formula that brought him back to life after his death, as it did for Norman, so one would think it would still be in his system in some capacity. A simple error by the writers, or a future store note? I suppose only time will tell. I have to say, though, that I really enjoyed this story. Was it padded? Sure, but it was entertaining padding, and the final two parts were absolutely awesome. I am very much looking forward to Doc Ock's next move, as we know he will be prominently involved in 'Big Time.' In general the spider books are very good right now, in my opinion.
I'm pretty sure the months Harry spent in Europe were to "detoxify him", not from drugs or alcohol but the formula. So I think he's normal now.