When can illegally obtained evidence be used to impeach a defendant's testimony?

Illegally obtained evidence can be utilized to impeach a defendant's testimony primarily during cross-examination. This is because the primary concern with such evidence is its ability to challenge the credibility of a witness, in this case, the defendant, without potentially transforming the trial into an exploration of the legality of the evidence itself.

The rationale is grounded in the notion of fair trial principles. When a defendant takes the stand, they open themselves up to scrutiny regarding their truthfulness and reliability. If evidence—regardless of how it was obtained—can demonstrate inconsistencies or contradictions in their testimony, it is permissible for the prosecution to use that evidence specifically in the context of cross-examination. This usage is aimed at exposing the truth and ensuring that the jury can properly assess the credibility of the defendant’s statements.

Using this evidence in other scenarios, such as during closing arguments or in the examination of other witnesses, would create significant legal and procedural complications, potentially infringing upon the defendant's rights and diverting the trial's focus from the substantive issues at hand.