Yup. It's not The Monster, but The Manster. Part Man, part Monster. This is an agreeably daft piece of low-budget mad scientist B-twaddle, interestingly co-produced with (and set in) Japan, although it's a Japan where everyone speaks English, with some laughable moments but generally moves at a fast enough pace to gloss over the arrant silliness and iffy make-up effects. Ultimately the worst that can be said about it is that the secondary female lead bears a terrifying resemblance to Edwina Currie; something which frankly scared me more than the Manster on one of its blood-crazed rampages.
There are actually two man-monster Mansters in The Manster: a giant ape creature savages some girls in a bath-house in rural Japan before returning to the mountaintop laboratory of its demented creator Dr Suzuki. Clearly the formula isn't working, so he changes the enzyme and selects a new test subject: he drugs newspaper reporter Larry (Peter Dyneley) with dodgy whisky and injects him with the new improved serum. But it's not long before Larry starts to transform into his inner beast: he loses his focus at work, dumps his long-suffering wife and takes up with Suzuki's glamorous assistant (Edwina lookalike Terri Zimmern in, as far as the IMDb is concerned, her only film appearance). His physical transformation starts with the hairy hand before developing an eye on his shoulder that becomes a second head! (Evil Dead 3: Army Of Darkness nodded to this at one point.) And then.....
The second head itself is a pretty terrible effect: it doesn't actually do anything except sit on the actor's shoulder (probably glued to his raincoat so it doesn't fall off when he runs) and is less convincing even than the one Zaphod Beeblebrox had in the BBC version of Hitch-hiker's Guide. But it is pretty much of the level you'd expect from a Z-list horror movie from 1959: clunky writing: out of nowhere a volcano starts erupting, and the final chunk of dialogue is almost literally unspeakable. Still, it's not supposed to be Citizen Kane. Idiotic, but not without a few moments of loopy charm. The Manster doesn't appear to have ever been released in the UK; this was seen on an imported DVD.