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Monster A Go-Go (also Monster A-Go Go and Terror at Halfday) is a 1965 science fiction movie directed by Bill Rebane (credited) and Herschell Gordon Lewis (uncredited).

An astronaut is about to be sent up into space, and is given regular doses of "Radiation Repellent" for a while before the launch. For no particular reason, just prior to the launch, the astronaut's doctor switches to a totally different Radiation Repellent which hasn't been tested as much. And, naturally, he does this without telling any of his superiors. And, well, the test that they did conduct resulted in an animal doubling in size, right before that animal died. As expected with these kind of results, they not only gave the new repellent to the astronaut, but doubled his dosage.

So, the astronaut's capsule crashes in an empty field. The astronaut, thanks to the untested repellent, is now freakishly huge and killing random people. Until, that is, one scientist locks him up in a storeroom for eight weeks and gives him doses of an antidote. And, again, he does this without telling any of his superiors.