Hard drugs have been defined as drugs that are biologically active and non-metabolizable in vivo. Soft drugs are defined as drugs, which are characterized by predictable and controllable in vivo destruction (i.e. metabolism) to form non-toxic products after they have achieved their therapeutic role. Quaternary ammonium compounds, such as benzalkonium chloride, are hard antibacterial agents. Their toxicity limits their usage in humans and animals, and their chemical stability limits their usage for general environmental sanitation. Furthermore, due to their stability they are prone to induce selective antimicrobial pressure and bacterial resistance. Soft analogs of the currently available hard antibacterial agents are less toxic. However, although the soft analogs have been shown to possess antibacterial activity in in vitro studies, it is likely that their in vivo activity will be hampered by their chemical instability.