  We address the impact of sterile fermions on the lepton flavor violating
decays of quarkonia as well as of the $Z$ boson. We compute the relevant Wilson
coefficients and show that the ${\rm B}(V\to\ell_\alpha\ell_\beta)$, where
$V=\phi,\psi^{(n)}$, $\Upsilon^{(n)},Z$ can be significantly enhanced in the
case of large sterile fermion masses and a non-negligible active-sterile
mixing. We illustrate that feature in a specific minimal realization of the
inverse seesaw mechanism, known as $(2,3)$-ISS, and in an effective model in
which the presence of non-standard sterile fermions is parameterized by means
of one heavy sterile (Majorana) neutrino.
