Multiplexers (MUXes) are ubiquitous in digital circuits. A MUX processes inputs a, b and a select bit s to output a if s = 0 and b otherwise. Hence, in the case of a = b, the output must be a, regardless of s. Unfortunately, common MUX implementations violate that specification if s is degraded. We propose efficient transistor-level implementations of Metastability-Containing Multiplexers (CMUXes). A CMUX behaves like a regular MUX, but additionally guarantees to output a whenever a = b, even if s is severely degraded. Our implementations require 8 and 10 transistors, respectively, and thus have no or little overhead w.r.t. the conventional 8-transistor MUX. We evaluate our implementations using SPICE simulations. Our CMUXes are a contribution to metastability-containing circuits in general, as they are a drop-in replacement for the naive implementations used to date; we demonstrate this by reducing the transistor count of the metastability-containing sorting network by Lenzen and Medina (ASYNC'16) and Bund et al. (DATE'17) by up to 69 % and 65 %.