As an American reader, I can only assume that the use of 'fag' here is short for 'fatigue,' and not an insinuation that Miss Elsie's brain might be batting for the same team, as it were.Actually, the fact that this wonder tonic cures not only 'brain fag' but 'premature decay' makes me worry. Did people from this era normally start decaying while still alive, instead of waiting till they were decently dead and buried? I realize life was difficult back then, but I didn't realize it was THAT difficult.
You're right, Amphigorym: how worrying! I was equally perturbed when I misread "heat nausea" as "meat nausea"... the idea of which makes me quite nauseous in itself.