The incidence of side-effects depends on the drug you're taking. The most common types (in Australia at least) are AII2 antagonists (eg. candesartan). ACE inhibitors are also up there. These have very few side effects in an otherwise well individual. If however you're on beta blockers (rare to start people on these these days), there are definite side effects and you would need to 'wash in' for a couple of weeks -- the side effects of these are very significantfor flying, if not extreme ie. drowsiness, fainting, light-headedness. So... what you'll experience depends on the drug. I'd hope the regulators via the AME's are up on this.