By MANSUR SHAHEEN DEPUTY HEALTH EDITOR FOR DAILYMAIL.COM PUBLISHED: 12:31 EST, 20 December 2022 | UPDATED: 13:44 EST, 20 December 2022 Two already-approved drugs may effectively treat the millions of Americans estimated to have long Covid. Guanfacine, an Attention Deficit/Hyperactive Disorder (ADHD) drug sold under the name Tenex, and N-acetylcysteine (NAC), a concussion drug that is branded Mucomyst...