Meta-stable dark sector particles decaying into electrons or photons may non-trivially change the Hubble rate, lead to entropy injection into the thermal bath of Standard Model particles and may also photodisintegrate light nuclei formed in the early universe. We study generic constraints from Big Bang Nucleosynthesis on such a setup, with a particular emphasis on MeV-scale particles which are neither fully relativistic nor non-relativistic during all times relevant for Big Bang Nucleosynthesis. We apply our results to a simple model of self-interacting dark matter with a light scalar mediator. This setup turns out to be severely constrained by these considerations in combination with direct dark matter searches and will be fully tested with the next generation of low-threshold direct detection experiments.
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