It happened at the Dandelion Dispensary, near9th Street and Walnut Street in Boulder, just before 7 p.m.
Police say the man walked into the dispensary and immediately sprayed what police say was bear spray, a potent form of pepper spray.
One person was transported to the hospital with unknown injuries.
The masked man, described by police as "not thin" and wearing all black, took off after spraying the repellent.
Police say he may have had an accomplice waiting outside.
Police say they are looking for a vehicle involved, possibly a Pontiac Grand Prix.
It took officers a while before they were able to enter the dispensary because ofthe lingering haze from the spray.