A standard category of frequency for adverse drug reactions are provided in "Guidelines for Preparing Core Clinical-Safety Information on Drugs" - Report of CIOMS Working Group III (1995).
Very common >= 1/10 (>= 10%)
Common >= 1/100 and <>= 1% and < 10%)
Uncommon >= 1/1000 and <>= 0.1% and < 1%)
Rare >= 1/10,000 and <>= 0.01% and < 0.1%)
Very rare < 1/10,000 (< 0.01%)
CIOMS working group recognizes it is always difficult to estimate incidence on the basis of spontaneous reports, owing to the uncertainty inherent in estimating the denominator and degree of under-reporting. However, whenever possible, the AE frequency should be provided.