Methods of improving patient safety and reducing undesirable side effects for patients considering therapeutic treatment using monomethyl fumarate and prodrugs of monomethyl fumarate are disclosed. In particular, a method of treating a disease in a patient in need of such treatment is provided. The method comprises testing the patient for a propensity for a deficiency in tissue glutathione S-transferase theta 1 enzyme (GSTT1) levels. Thereafter, a therapeutically effective amount of a compound selected from monomethyl fumarate (MMF), a prodrug of monomethyl fumarate, and combinations thereof is administered to the patient. During the treatment of the disease, blood lymphocyte concentration is periodically tested in the patient at a predetermined time interval length that is based on the enzyme level propensity testing result.
