Evil Spawn (1987)

The film opens with narrative that a spaceship has returned to Earth carrying alien microbes. This is never mentioned again.  Perhaps they were part of an experiment gone wrong?  But honestly when science goes wrong, it's more likely something other than aliens, such as evil.

Dr Zeitman, played by John Carradine (in footage culled from footage shot separately from the rest of the film) is poisoned by his assistant after offering aging actress Lynn Roman a chance to become young via experimental injections.  Desperate to get a leading role in an upcoming blockbuster, Lynn decides to inject herself with the serum. Unfortunately the Doctor lied.  It's actually an experiment in cellular mutation which turns humans into monsters who eventually morph into giant bugs. Why would anyone want to do that?  Seems like a rather useless experiment.

Lynn regains her human form when the serum wears off.  But with another injection, the cellular instability causes her transformation again.  Even when the serum causes her to look younger between turns as a hideous monster, the producer won't hire her because she's too old for the job. Serves her right for taking monster serum.

Her agent represents a young actress who gets the role and her boyfriend is secretly seeing a younger woman who he brings to their house.  Why?  Why?  No idea.  Needless to say the old, raging, bug-like hag is not happy to see some young thing in her house messing around with her man and carnage ensues.