11:19 AM PT -- Manziel addressed the incident on social media, saying ... "Thank you everyone for your concern and kind messages. Unfortunately I had a reaction to an increased dosage in Lithium which I take for my Bipolar disorder."

"It was a scary moment and I'm especially grateful for the staff at the hospital and all that they've done in the last 24 hours."

Johnny Manziel was hospitalized Monday night in Texas, TMZ Sports has learned.

Sources connected to the QB tell us ... he was admitted to a hospital in Humble, TX on Monday. He had been in the area to attend the JMBLYA concert with friends after playing in a golf tournament hosted by friend and former Texas A&M teammate Mike Evans.

Manziel's rep, Denise Michaels, tells us ... "It seems like he had a reaction to an adjustment in his prescription. He’s fine and headed home."

Back in February, Manziel appeared on "Good Morning America" and revealed that he suffers from bipolar disorder.

Unclear if the medication that landed him in the hospital is connected to that diagnosis.

The 25-year-old has been working on an NFL comeback over the last several months -- and according to NFL insiders, he's been doing well ... impressing scouts and team execs during workouts and Spring League games.