Fifty-five people were treated for injuries and illness, including six for gunshot wounds, during clashes between rival demonstrators outside parliament on Tuesday.
Seven riot police also tested negative for drug use after some crystal methamphetamine was found in a police van.
Police claimed the drugs were planted in the vehicle, which had been surrounded and damaged during the confrontation.
The van's windshield was broken and some windows removed, and it was also sprayed with paint. Items that officers had left in the van were also missing.
Police were filing a complaint against people who distributed "fake news" regarding the drugs in the police van.
Photos of the drugs in the police vehicle were widely shared after an opposition MP posted them on social media on Tuesday.
Police insisted they had followed legal procedures in handing the protest. He said no rubber bullets or live ammunition had been prepared for the operation
Bangkok's Erawan Medical Centre said a total of 55 people were hurt, six of them for gunshot wounds and 32 for the effects of tear gas.
Four others fell ill and the 13 others had other injuries. They were treated at nine hospitals.
