Aside from the fact Rigsby was willing to get his prescription from Dr. Harper filled out even though he knew she wasn't really a doctor, the pharmacy would have been unable to fill it. As it was shown, the prescription only had the name of the drug on it (the generic name for Tamilflu), but it didn't have the dosage information on it (the milligram amount, how many pills, etc ) making it impossible for them to fill it. And then when he's seen next (on the stakeout with van Pelt) he is suddenly cured without showing any of the symptoms he'd been suffering from that morning.