2:05 PM PT -- Jake's attorney, Larry Stein, tells us, "We take this claim very seriously, and are working with authorities to make absolutely sure we do our part to uncover the truth."

Jake Paul hosted a massive party for Desiigner's birthday, and one female guest says things got so out of control ... someone drugged her, and now cops are investigating.

Law enforcement tells TMZ ... officers were contacted Sunday about a possible case of unwillful impairment that occurred Saturday night at the YouTube star's mansion in Calabasas.

We're told cops are looking into whether the alleged victim -- a female in her early 20s -- unknowingly took something that severely impaired her. The investigation is in its early stages, and law enforcement is asking anyone with information to reach out to the L.A. County Sheriff's Department.

Important to note ... neither Jake nor Desiigner is accused of anything. We reached out to Jake's reps, but no word back.

Our sources say paramedics made multiple house calls during the party, one shortly before midnight for a sick person transported to the hospital. Another call was made for the same thing after 1 AM, and a third call happened around 3 AM for an adult who fell and got injured.

As for the party ... Jake's celebration for Desiigner's 22nd bday had a ton of celeb guests, including Chris Brown, Lamar Odom, Bella Thorne, Swae Lee, Justin Combs, Paris Brosnan, Trey Songz, Famous Dex and Cameron Dallas. The party featured live boxing matches and motorcycle stunts.

As a result ... we're told there were private ambulances and paramedics in the driveway of Jake's home, at the ready if anyone needed medical treatment. Still, the 911 calls were answered by county firefighters, not the private ones.

Additionally, sources involved tell us security at the party was supervised by a retired Sheriff's deputy, and they had 38 guards on site to check IDs.