Innerspace (1987)

Tuck Pendleton is a cocky pilot, who is taking part in a miniaturization experiment. When some bad guys break into the lab to steal the technology, one of the scientists takes a syringe which contains the miniaturized Tuck and vessel. Now in the vessel is part of the material needed to restore him. But the other part which is the lab is stolen. The scientist’s shot but before dying he injects Tuck into Jack Putter, a hypochondriac, who feels that something is wrong with him all the time. When Tuck links himself to Jack’s systems, he discovers that something happened. So they go back to the lab, and discover what happened. 

Credit Warner Bros