Using alternating electric fields to increase cell membrane permeability

A method for treating a tumor in a subject's body and delivering a substance across cell membranes in the subject's body, the method comprising:

• applying a first alternating electric field at a first frequency to the subject's body for a first period of time, wherein application of the first alternating electric field at the first frequency to the subject's body for the first period of time increases permeability of the cell membranes in the subject's body;
• administering the substance to the subject, wherein the increased permeability of the cell membranes enables the substance to cross the cell membranes;
• and
• applying a second alternating electric field at a second frequency to the subject's body for a second period of time that is at least one week long, wherein the second frequency is different from the first frequency, and wherein the second alternating electric field at the second frequency inhibits growth of the tumor.