It can also be used in plural form (large quantities of ) but as subject it always takes a singular verb; as
During the raid, large quantities of cocaine [STRIKE]was [/STRIKE]were seized at Beirut port
The plural form is used in phases like: in large/small/sufficient etc quantities; as:
Green tea can be harmful if taken in large quantities
Glucose is used in large quantities for medical purposes
Buy vegetables in small quantities, for your immediate use.