I remember being quite young, so I would have seen it on television in the early 90's. It was a film about a man who developed a condition (can't quite remember the circumstances that brought about the situation) where a second head would grow and replace his own out of his shoulder, it slowly started to take over his life. It was all very strange. That's all I can remember unfortunately, so not a lot to go on.
I think it is "How To Get Ahead In Advertising"
Dennis Dimbleby Bagley is a brilliant young advertising executive who can't come up with a slogan to sell a revolutionary new pimple cream. His obsessive worrying affects not only his relationship with his wife, his friends and his boss, but also his own body - graphically demonstrated when he grows a large stress-related boil on his shoulder. But when the boil grows eyes and a mouth and starts talking, Bagley really begins to think he's lost his mind
There is a good summary here
Though I'm not sure if it counts as sci-fi.
This is How to get ahead in advertising
Richard E Grant plays an advertising exec who becomes convinced that an ugly boil on his neck speaks to him. Eventually the boil grows to be another head and
His actual head shrinks, the boil taking over his body
There was a movie that I saw in the eighties called "The Boil." My friend and I still laugh about it because it was so bad, but other movies may have copied the premise.