there are drugs that metabolize so fast they cannot be detected. I think halothane is one. and I'm not sure that coroners test for every single possible drug.
Also, I'm not sure if its true for a drug screen on a dead body, but I know in employment drug tests, if you have a prescription for something it will not be listed on the test as a positive. It will either say negative or be a blank. for example, if jb was taking something like benedryl regularly for allergies (I think I read somewhere she had sinus/allergy issues), there would be no cause for alarm to find dyphenhydramine in her system. did the report say absolutely no drugs found, or was it along the lines of no drugs that have no reason to be in her system were found? I'm thinking the ramseys must have had a reason to imply that the white's poisoned jb,,,,the reason being that jb was given something by the parents, even if just a couple of benedryl to make her really sleepy. of course, that would mean they planned the murder, however I believe it occured during a rage. I'm just thinking while typing...