Valtrex is an antiviral drug which was shown in a pilot study to improve mental symptoms and fatigue after taking it for six months. The improvements lasted three months more after patients stopped taking it but longer outcomes data isn't available. Mitochondrial damage is a hallmark of CFS. One concern I have about Valtrex is that it has been found to prevent mitochondrial reproduction. Over time, this should lead to a lowering of the total number of mitochondria in affected cells and a worsening of CFS fatigue symptoms. “Mitochondrial DNA synthesis in Molt-4 cells was particularly sensitive to ddC. The ratio of mitochondrial to cellular DNA was decreased 80% after 5 days of treatment with 0.05 puM ddC; however, there was no apparent increase in the doubling times of the treated cells. (Martin, JL. Antimicrob. Agents Chemother. 1994, 38(12):2743)."