The Happy Fast

How sure are you that what you're seeing is real? After the third world war finished with the US destroying 2/3 of Asia, the world was in turmoil. An emergency plan put in place by the UN to create a mind control drug to make the surviving population happy, causes a severe side affect in a few test subjects who begin to see a different reality from everyone else. The drug is leaked and soon, total chaos breaks out as The Happy Drug drives people to suicide.
With the US closing its comms and starting a space race with Italy; Japan, recovering from radiation leaks; Canada, Austria and the UK, running human trials of the drug and refugees flooding in across the world with a diminishing population, who will survive and
How sure are you now?