There aren't many Boots outlets in the middle of Afghanistan; where you'd be expected to fly in an unpressurised aircraft above 10 000ft. If you've never experienced the effects that blocked sinuses have on you in a descent, then you don't want to, believe me. Trust me, hayfever affects flying.
The issue isn't drowsiness, it's the effects of mucus buildup through your system; sneezing, blockages etc. It may be perfectly treatable with tablets, but they can't guarantee to have them available on deployments, hence you're undeployable.