  We analyse effective potential around the electroweak (EW) scale in the
Standard Model extended with a heavy scalar doublet. We show that the
additional scalars can have a strong impact on vacuum stability. Although the
additional heavy scalars may improve the behaviour of running Higgs
self-coupling at large field values, we prove that they can destabilise the
vacuum due to EW-scale effects. A new EW symmetry conserving minimum of the
effective potential can appear rendering the electroweak symmetry breaking
minimum meta- or unstable. However, for the case of the inert doublet model
(IDM) with a 125 GeV Higgs boson we demonstrate that the parameter space region
where the vacuum is meta- or unstable cannot be reconciled with the constraints
from perturbative unitarity, electroweak precision tests and dark matter relic
abundance measurements.
