An intriguing question, to which, presumably, only she knows the answer.
More pertinently imo - why didn't the PJ get them checked out. If it was proved that drugs had been administered then surely that would be vital evidence - and because of that I don't think the McCanns could have had checks done themselves - IMO it could only done under the auspices of the PJ - as part of an ongoing enquiry.
Quote from KM's book
''Since Madeleine was snatched apparently without making a sound, we had always suspected that all three children might have been sedated by the abductor. We mentioned this to the police that night and several more times in the following weeks, but no testing of urine, blood or hair, which could have revealed the presence of drugs, had ever been done.''
Unquote
A report from a FLO also confirms that the McCanns asked the PJ about the possibility of drugs having been used.
However, IIRC it was the PJ's decision not to do the tests.