I hardly saw this as a "horror" in say, the fangy Hammer or dirty Corman traditions, but more of a internal horror of addiction and ambition. Karloff's performance is a class act, even if some of the bit players don't rise to his occasion. The production values, especially around the seedy Seven Dials, is perfectly scummy.
Those expecting the film to live up to its lurid "Hammer Horror" style title will be disappointed; this is essentially a medical melodrama 'mis-sold' as cheap exploitation. While probably too staid for most viewers, the film is well acted, quietly compelling & benefits from a gritty steam-punk recreation of 19th century London that may have possibly influenced Lynch's depiction of the same era in The Elephant Man.
B-movie is not so much a horror film as a dark period melodrama with some surprisingly gruesome touches for its era. A strong performance from horror icon Boris Karloff, and an effective atmosphere - director Robert Day makes great use of his low budget - but it's slow-paced and the story isn't too compelling. Well-made, but not particularly memorable.