A cough drop is a rough ellipsoid of 2cm x 1.6cm x 1cm (just got a pair of calipers from Amazon)
4/3pi*1*.8*.5 ~= 1.6cm^3
each cough drop is 10 calories
a gallon is about 3785 cubic centimeters
assuming packing density of 45%, that's 1064 cough drops, or 10640 kCal

but wait - wikipedia says median lethal dose of menthol is 192mg/kg
i weigh 65 kg
that's 12480mg of cough drop menthol needed to kill me
1064 cough drops * 7.5 mg menthol = 7980 mg

GOCDAD is a go