Many years back, my wife was the only MD on a flight responding to a 'any doctors on board' -- she evaluated the passenger and determined the matter was not 'critical' -- more a matter of dehydration caused by perhaps a bit too much alcohol (after doing basic health checks).

This was pre 2001 so she got called up to the cockpit to brief the pilot. He asked if they needed to land quickly (at the time they were two hours out of the scheduled landing). She indicated the passenger was resting comfortably (my wife and the passenger were relocated to the 1st class cabin) and a early landing wasn't required. She related that she repressed the urge to tell the pilot (who had turned to face her) to keep his eye on the road. <smile>.

But yes, my impression is that medical issues are taken very seriously.