Veratril was a drug Mahone used to prevent the traumatic visons of Oscar Shales. He developed an addiction to Veratril, If he didn't use any for more than 3 days then his mental health will become unstable. In the episode, Good Fences, Mahone experienced paranoia, spasms, agitation and delusions of Charles Patoshik, because he couldn't resupply when he was arrested in Panama. Veratril (referred to as Diazepam in S2:E19 "Sweet Caroline") cause the sensation of "walking underwater" (Mahone).
The pen Mahone stored veratril in.
Mahone finally breaks his addiction to Veratril while in Sona.
Mahone dispenses a veratril pill from his pen