Milo Yiannopoulos -- Ye's chief of staff for two years before departing the company earlier this year -- submitted an affidavit to the California Dental Board claiming Ye's dentist, Thomas P. Connelly, provided Kanye with nitrous oxide for recreational use.

Yiannopoulos claims Connelly provided the gas for cash ... taking in upwards of $50K every month, even when the gas led to distressing signs from Ye -- including addiction to the substance.

Milo says Ye talked about nitrous oxide nonstop in meetings, appearing in an inhaler mask consistently. His direction to staffers allegedly became incomprehensible and contradictory, and his mood unpredictable.

A Yeezy employee saw Kanye self-administering, according to Milo, and he says it's unclear if anyone was using a regulator.

Connelly's firing back, however ... with his spokesperson telling TMZ he's denying all allegations leveled against him by Milo -- calling the narrative "not only factually incorrect but also intentionally misleading."

TC's rep says Milo's making up these claims for attention ... and is attempting to maliciously damage Connelly's reputation in the process.

The Connelly camp ends its statement saying Dr. Connelly adheres to all ethical and legal standards and is committed to providing the highest standard of care to his patients and will continue to do so despite these unfounded attacks on his character and professional conduct.