Inclusion Criteria:

1. Advanced cancer patients seen in outpatient clinics or inpatient units at MD Anderson Cancer Center, with presence of anorexia for at least one month, and accompanied by weight loss of > 5 % of pre-illness body weight in the last 6 months. Anorexia on the day of enrollment (day 0 +/-3) must be > 4/10 on ESAS.
2. Patients > 18 years of age
3. Karnofsky Performance score of > 40 at time of inclusion into study
4. Ability to provide informed consent and comply with study procedures
5. Ability and willingness to return to engage in telephone follow-up by research nurse on days 2 (+/- 3 days), 8 (+/- 3 days), 16 ( +/- 3 days), and 22 (+/- 3 days) and return to outpatient clinic for evaluation on days 15 (+/- 3 days), and 29 (+/- 3 days).
6. Negative urine pregnancy test at time of inclusion into study for female patients of childbearing potential, within 24 hours of study enrollment.
7. For patients receiving chemotherapy eligibility to be determined after discussion with primary oncologist
8. Drowsiness of </= 4/10 on ESAS at baseline due to known potential sedating effects of Mirtazapine
9. Patient can continue all medications including complementary therapies or antineoplastic therapy while on-study (with the permission of their Attending MD) if they have been on stable dose for at least 2 weeks.
10. Patients who are on stable doses of antidepressants for at least 2 months.

Exclusion Criteria:

1. Patients who have known dementia or delirium at time of enrollment as determined by a physician.
2. Known hypersensitivity to any ingredient of Mirtazapine
3. Inability to maintain oral intake over the course of the study, such as with mechanical obstruction of the alimentary tract or intractable vomiting
4. Ongoing use of tube feeding or parenteral nutrition
5. Current use of corticosteroids (with the exception of scheduled doses during time of chemotherapy), dronabinol, testosterone and progesterone derivatives such as megestrol acetate, or in the preceding 1 week prior to study enrollment.
6. Pregnancy or lactation or unwillingness to use contraceptives
7. A score of 11 or more, in each subscale of the Hospital Anxiety and Depression scale (HADS) indicating clinical depression or anxiety. Those with HADS scores of > 11 at baseline indicating moderate or severe depression will be excluded from the study and will be referred for appropriate follow up by counselor and psychiatry evaluation.
8. Patients on chronic use of benzodiazepines are excluded