If sleeping pills had been ingested,Steve,,they'd have shown up in tests,,,particularly when there weren't any stomach contents as well,,so you can cross those off your list.
The only one taking noxious substances was Sheila,,,cannabis,cocaine and amphetamines,along with her prescribed medication,,to which if she was indulging in the aforementioned,then my guess is that she wouldn't have also been taking her prescription medicine.