  We derive the general resummed effective Lagrangian for Higgs-mediated
flavour-changing neutral-current (FCNC) interactions in the Minimal
Supersymmetric Standard Model (MSSM), without resorting to particular
assumptions that rely on the squark-mass or the quark-Yukawa structure of the
theory. In our derivation we also include the possibility of explicit CP
violation through the Cabibbo--Kobayashi--Maskawa mixing matrix and soft
supersymmetry-breaking mass terms. The advantages of our resummed FCNC
effective Lagrangian are explicitly demonstrated within the context of
phenomenologically motivated scenarios. We obtain new testable predictions in
the large $\tan\beta$ regime of the theory for CP-conserving and CP-violating
observables related to the $K$- and $B$-meson systems, such as $\Delta
M_{K,B}$, $\epsilon_K$, $\epsilon'/\epsilon$, ${\cal B}(B_{d,s} \to
\ell^+\ell^-)$ and their associated leptonic CP asymmetries. Finally, based on
our resummed FCNC effective Lagrangian, we can identify configurations in the
soft supersymmetry-breaking parameter space, for which a kind of a
Glashow--Iliopoulos--Maiani-cancellation mechanism becomes operative and hence
all Higgs-mediated, $\tan\beta$-enhanced effects on $K$- and $B$-meson FCNC
observables vanish.
