Former NFL quarterback Johnny Manziel was hospitalized Monday for a "reaction" to a prescription.

On Instagram, Manziel posted: "Thank you 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."

Denise Michaels, a Manziel spokesperson, told ESPN's Kevin Seifert on Tuesday that her client was "fine."

According to TMZ Sports, Manziel was hospitalized in Humble, Texas, on Monday night.

After being released by the Cleveland Browns in March 2016, Manziel plunged into a spiral of substance abuse that he often chronicled on social media. He was diagnosed with bipolar disorder and has been treated for depression.

Manziel, 25, won the Heisman Trophy at Texas A&M in 2012. He is attempting a comeback in professional football. He played in the developmental Spring League in April and has a standing offer from the CFL's Hamilton Tiger-Cats.

ESPN's Kevin Seifert contributed to this report.