I vividly remember a movie from my youth that scared me so much I had to call mom to help me watch the rest of it.
I just tried to find the title and info about it but failed.
I thought it was THE THING WITH TWO HEADS with RAY MILLAND but the synopsis says that a doctor transplanted a head onto a guy.
The movie I saw has a guy growing a second head. It started as an eye growing in his shoulder and grew into a complete second head! (I thing the guy splits in two later in the movie.)
The scary part, for me, was the “monster” behind the door! He shook the door so violently in the evil doctor’s laboratory that it scared me.
UPDATE: NAME THAT TRAUMA SOLVED! Big ups to Name That Trauma champ Senski who knew that this was from THE MANSTER.