The M2 proton channel of influenza A virus is an integral membrane protein involved in the acidification of the viral interior, a step necessary for the release of the viral genetic material and replication of new virions. The aim of this study is to explore the mechanism of drug (un)binding to the M2 channel in order to gain insight into the structural and energetic features relevant for the development of novel inhibitors. To this end, we have investigated the binding of amantadine (Amt) to the wild type (wt) M2 channel and its V27A variant using multiple independent molecular dynamics simulations, exploratory conventional metadynamics, and multiple walkers well-tempered metadynamics calculations. The results allow us to propose a sequential mechanism for the (un)binding of Amt to the wt M2 channel, which involves the adoption of a transiently populated intermediate (up state) leading to the thermodynamically favored down binding mode in the channel pore. Furthermore, they suggest that chloride anions play a relevant role in stabilizing the down binding mode of Amt to the wt channel, giving rise to a kinetic trapping that explains the experimentally observed pseudoirreversible inhibition of the wt channel by Amt. We propose that this trapping mechanism underlies the inhibitory activity of potent M2 channel blockers, as supported by the experimental confirmation of the irreversible binding of a pyrrolidine analogue from electrophysiological current assays. Finally, the results reveal that the thermodynamics and kinetics of Amt (un)binding is very sensitive to the V27A mutation, providing a quantitative rationale to the drastic decrease in inhibitory potency against the V27A variant. Overall, these findings pave the way to explore the inhibitory activity of Amt-related analogues in mutated M2 channel variants, providing guidelines for the design of novel inhibitors against resistant virus strains.