The parameters of the Higgs potential of the minimal supersymmetric standard model (MSSM) receive large radiative corrections which lift the mass of the lightest Higgs boson to the measured value of 126 GeV. Depending on the MSSM parameters, these radiative corrections may also lead to the situation that the local minimum corresponding to the electroweak vacuum state is not the global minimum of the Higgs potential. We analyze the stability of the vacuum for the case of heavy squark masses as favored by current LHC data. To this end we first consider an effective Lagrangian obtained by integrating out the heavy squarks and then study the MSSM one-loop effective potential , which comprises all higher-dimensional Higgs couplings of the effective Lagrangian. We find that only the second method gives correct results and argue that the criterion of vacuum stability should be included in phenomenological analyses of the allowed MSSM parameter space. Discussing the cases of squark masses of 1 and 2 TeV we show that the criterion of vacuum stability excludes a portion of the MSSM parameter space in which and are large.