We next assessed the synergistic activity of double and triple combinations of amantadine, ribavirin, and oseltamivir carboxylate over a range of concentrations of each drug against each 2009 H1N1 isolate. A quantitative measure of the total synergy (or antagonism) of a drug combination can be expressed in terms of synergy volumes, which represents the cumulative synergy and antagonism across all concentrations for all the drugs in a combination. Based on the empirically determined criterion of synergy volume .100 mg/mL 2 % using the lower confidence interval, all three double combinations were additive against the 2009 H1N1viruses (Table 1) . By contrast, the TCAD regimen was synergistic against all three viruses over multiple concentrations of all three drugs, with the synergy occurring at 0.1 mg/mL and above for amantadine, 0.32 mg/mL and above for ribavirin, and 0.0032 mg/mL and above for oseltamivir carboxylate (Table 1) . These data show that the synergy of the TCAD regimen occurred at clinically achievable concentrations for all three drugs, given that the expected average plasma concentrations based on the recommended doses are 0.43 mg/mL for amantadine, 1.3 mg/mL for ribavirin, and 0.3 mg/mL for oseltamivir carboxylate. Furthermore, the synergy of the TCAD regimen was greater than the synergy of any double combination tested for all three 2009 H1N1 strains. Figure 1B -1D shows the synergy of the TCAD regiment as a function of increasing concentrations of amantadine, ribavirin, or oseltamivir carboxylate as the third drug, representing the contribution of each drug to the synergy of the double combination without the third drug. These data reveal that the addition of each drug as the third drug to the double combinations resulted in a concentration-dependent increase in synergy, indicative that each drug contributed to the synergy of the TCAD regimen.