Doctors Say It Was Probably The Cold Medicine

A piece on MedPage Today by Jeffrey Sonnenfeld, DBA, MBA, and Harlan Krumholz, MD, SM entitled, "Did Cold Medications Affect Biden's Debate Performance?" seems to answer the questions:

In response to a question about abortion, Biden hesitatingly offered jumbled canned lines about immigration while staring dazed, mouth agape during breaks. This is not the president we have seen in the last 3 years or even the last few weeks.

As time passed, the fog lifted. He improved throughout the debate and later in the evening...