Police in the eastern province of Anhui arrested eight gang members, alleging they sold 200,000 of the syringes throughout China filled with a large dose of the muscle relaxant suxamethonium.
The buyers were mainly dog vendors who collect and sell dogs to restaurants for meat, the Xinhua news agency said, citing police who warned that people who ate the meat were also in danger of being poisoned.
The needles were modified by the gang with a spring and tailfin so they could be shot from a distance like a dart.
Police said that the hunt was on to find more of the syringes, which contained enough suxamethonium to kill the animals immediately.
When police raided the gang's lair in Enshi City, in central Hubei Province, in October, they found four kilos of chemical powder, 10,000 needles and 100,000 yuan ($15,000).