The pro-guilt camp assert Sheila was sedated and refer to page 3 of Dr. Ferguson's statement of 18th. September 1985.
I am not a psychiatrist or a pharmacologist, but I do NOT accept Dr. Ferguson's opinion on the point. I think he is likely to be wrong when he says that recreational drugs can have no impact on the effect of anti-psychotics.
It is important to recognise that a psychiatrist is not an expert on psycho-pharmacology and does not necessarily have anything more than a working knowledge of how drugs work. I think Dr. Ferguson's view in that paragraph is obviously flawed and wrong and I think it would be easy to show this.
Indeed, I see that in sheet 4 of his own statement of 8th. August 1985, Dr. Ferguson clearly states that the use of illicit drugs would exacerbate Sheila's psychosis, which is inconsistent with his later claim.
I find that quite suspicious, actually. The first claim seems right. The second claim, made after Jeremy became a suspect, seems wrong but convenient for the police.
Intellectual fallacy: Argument from authority. Having a title does not in and of itself mean you are correct.
However, an argument from authority is not always fallacious. It would not be a fallacy to rely on Dr. Ferguson if it could be shown that Dr. Ferguson had relevant expertise, and that has not been shown. Psychiatry is a very learned profession, but it only implies a working knowledge of the impact of anti-psychotics. Psychiatrists are not pharmacologists. Moreover, the view of a clinical psychiatrist might be clouded by loyalty to a patient - even though they are professionals, psychiatrists are still human like the rest of us.
Thus, in that statement of his, Dr. Ferguson advances a view outside his strict expertise. You're simply relying on his title and you're ignoring the inconsistency in his evidence.
It's also noted that Dr. Ferguson only makes a general comment on the effect of cannabis on Haloperidol, he fails to mention its effect on Sheila's use of Haloperidol, which is not really the same question.