It's Always Sunny In Philadelphia: 10 Most Insane Fan Theories

This theory seeks to explain why The Gang seem to be getting progressively worse and worse as the years go by. It claims that they are all slowly being poisoned by carbon monoxide.

This would make a lot of sense, as not only has their behaviour been getting worse throughout the years, but it seems that their health both physically and mentally has deteriorated as the series progresses. Except for Mac, they all seem to be in worse physical states.

Additionally, we know for a fact that this is abundantly possible from the evidence that we received in the episode Charlie Work. In that chapter of Paddy's Pub, it is revealed during a health inspection that Charlie takes out the batteries of the carbon monoxide detector to kill all of the rats in the basement.

It becomes evident that the basement is completely filled with the deadly stuff, and it could easily show that even if Charlie were to turn the detector back on, it might already be too late.

The thought that The Gang are all slowly dying seems like the kind of direction that the writers would make canon.