Rory Stewart, one of the (many) Tory leadership hopefuls has said that he smoked a little opium at a wedding in Iran but 'It had no effect on me because I was walking 25-30 miles a day at the time.'
I shall bear this in mind.
Next time the GP tells me I drink too much I shall reply on the same lines 'Two pints of cider at lunchtime and two more in the evening have no effect if I've walked 15-20 miles on the the day, so let's discount those from any count of units, shall we?'
In truth the lunchtime ones just make me slightly reckless with map-reading uncertainties.... a bit more inclined to gamble and press on.