An American Werewolf In London (1981)

Two American friends David & Jack and taking a walking holiday through Northern England and are attacked by a Werewolf. Jack is ripped to shreds and David survives but is badly scarred. David wakes up a week later in a hospital in London and is upset to hear of his friends passing. He begins to have nightmares and visions of running wild through the woods. He becomes very close to one of the nurses, Alex, and when discharged he stays with her in her flat in London. He begins to get a visit from his dead friend Jack who warns him that he will become a Werewolf at the next full moon and must take his own life. Thinking he's crazy he ignores all of Jacks warnings. Sure enough he transforms when the moon is full and goes on a murderous rampage through London.

IMDb: https://www.imdb.com/title/tt0082010