Marilyn had an empty stomach as she had eaten nothing that day and therefore the pills would’ve been absorbed quickly leaving no residue. Most pills take 30 minutes to absorb, while
on an empty stomach it would be even less.
Also, as a habitual user the drugs could have absorbed even more quickly as she developed a faster metabolism for them. Dr. Wecht states in Tales From The Morgue:
“ I just don’t see any problem with the fact that the drugs were not there. Again, when we have a stomach free of food, the drugs are going to be absorbed more rapidly into the system.”