The following table is provided as a guide only to the timeframe that should elapse between a medical event and the intended flight. The timeframes may be changed following considered medical assessment of a specific case (2):
Diagnosis  | Assessment required by a doctor with aviation medicine experience  | Accept as fit to fly if:  | Comments  | 
Otitis media and sinusitis  | Acute illness or with loss of Eustachian function  | If able to clear ears  | |
Tonsillectomy  | 10 days or less  | Although it may be ok to fly between day 3 and 6, there is a significant risk of bleeding between day 1 and 2 and between day 7 and 10  | |
Middle ear surgery  | 9 days or less  | >= 10 days with medical certificate from treating ENT  | Ex: Stapedectomy  | 
For up to date advice then check current guidance (1,2).
Upper respiratory tract infection - relative contraindication to flying (3)
Dental surgery - flying is contraindicated for 2 to 5 days (4)
Note that these are only guidelines and each airline has its own regulations and medical standards.
Reference:
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