In Ch. 9, AKA "Dr. Lanyon's Narrative," Dr. Jekyll has written to his long-time friend, Dr. Hastie Lanyon and asked him to run an errand for him that is apparently of grave importance. Landon does as instructed, and later Edward Hyde shows up at his home to collect the items Lanyon was to have for him. It is at that point that, after a brief interaction, Hyde drinks the potion and transforms back in Jekyll. If Jekyll had simply told Lanyon about this he would not have believed it. Even Jekyll's letter was looked upon with skepticism:

"Upon the reading of this letter, I made sure my colleague was insane; but till that was proved beyond the possibility of doubt, I felt bound to do as he requested" (Stevenson).

The only way to really convince Lanyon that Jekyll was capable of turning into Hyde and that Jekyll WAS Hyde, was to transform in front of him. Sadly, the shock of seeing this play out in front of him was just too much Dr. Lanyon to take, and he died not long after.