OK, how about avoiding the cocktail of assorted nasties found in shop bought cough syrup by making your own?
(Camera pans around the houseboat kitchen and lights upon an onion, some brown sugar, the other leg of the antiseptic-making tights and a washed out jar on the sink...)
Peel the onion and cut into flat slices.
Layer it in the jar with equal thickness layers of the sugar (a bit like making a lasagne) and leave for 24 hrs.
By then the juice in the onion will have mixed with the sugar to make a syrup.
Strain the resulting goop throught the foot of the tights into a suitable container.
Store it in the fridge, first removing Jack's store of batteries.
The flavour is a little... unusual...
but it makes for good cough syrup!