We investigate the cosmological stability of light bosonic dark matter carrying a tiny electric charge. In the wavelike regime of high occupation numbers, annihilation into gauge bosons can be drastically enhanced by parametric resonance. The millicharged particle can either be minimally coupled to photons, or its electromagnetic interaction can be mediated via kinetic mixing with a massless hidden photon. In the case of a direct coupling, current observational constraints on the millicharge are stronger than those arising from parametric resonance. For the (theoretically preferred) case of kinetic mixing, large regions of parameter space are affected by the parametric resonance, leading at least to a fragmentation of the dark matter field, if not its outright destruction.
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