I've done this quest innumerable times, I always drink the skooma because I prefer to progress the quest that way.
I know you can pick the locks but the skooma is my preferred means of infiltration and it works every time.

That being said, on my latest character after drinking the skooma and awakening in the cell I'm unable to move and the vampires outside do nothing.
Half the time when I try the vampires freak out asking "is someone there?" before necro-ing the dead addict in the cage and attacking me while I can't move.
I'm unable to find a workaround online, does anyone here know why this keeps happening? I've reloaded past saves and tried again and again to no avail.