Here’s a case discussing access to mental health records of a primary prosecution witness.
This was a due process and confrontation case. Here, as is not an infrequent issue, the prosecution succeeded in having damaging information about their witness excluded. The prosecution then went on to give an “incomplete and inaccurate picture” of their witness. The prosecution did this knowing full well that they were presenting a misleading picture. (Why that’s not prosecutorial misconduct I have no idea. [N.1])
The majority held that the Confrontation Clause was violated by the restrictions on cross-examination about the informant’s mental health and use of prescription medication. The jury was deprived of evidence concerning his ability to perceive and recall what transpired and the informant’s credibility. On this point, the majority noted: