 Why is Macklin's reaction to the shot alarming?
But Macklin is exactly what we need for a quick, dramatic test. "No, Harold, it isn't," Macklin admitted. "I would have said the common cold," Macklin replied, "but I suppose from what you have said you mean headaches." "Aspirins would still be used to reduce fever and relieve muscular pains," Mitchell said. "There are a number of different causes for headaches—nervous strain, fatigue, physical diseases from kidney complaints to tumors, over-indulgence—but there is one effect of all of this, the one real cause of headaches," Mitchell announced. "When your heart stops, your blood pressure goes to its lowest point," Mitchell said.
 The shot has somehow removed his intelligence.
 He's acting as if he took a narcotic, enough that Mrs. Macklin suspects that they gave him heroin?  He's much too happy, as observed by Sidney. He's inexplicably healthy and too adjusted.  He seems unbothered by it, despite the fact that it should have changed his life.  The shot has somehow removed his intelligence.