If your food intake has not decreased or your energy output increased, a sudden increase in appetite can be due to emotional factors, medications or medical conditions. However, to determine the cause you will also need to consider your other symptoms.
Pregnancy and PMS
Pregnancy can result in cravings, nausea or dramatic appetite increase. To benefit from early prenatal care, get tested quickly if pregnancy is a possibility. Premenstrual syndrome (PMS, or symptoms associated with the menstrual cycle) may also cause sudden appetite increase.
There are medications, such as dronabinol and megace, which are intended to help increase the appetite in patients who need to gain weight. Other drugs, such as amitriptyline and other anti-depressants, steroids and cyproheptadine, may have increased hunger as a side effect.
All types of diabetes may cause sudden or extreme hunger according to MayoClinic.com. EndocrineWeb.com lists sudden hunger as a sign of hyperthyroidism, which is an overactive thyroid that produces excess amounts of the hormone thyroxine.
Hypoglycemia, or low blood sugar, can cause sudden hunger. It is important to determine what is causing hypoglycemia as it is a symptom of other conditions such as diabetes.
Stress, anxiety, postpartum depression and depression are among the mental conditions that may carry the symptom of a sudden increase in appetite.
Cyproheptadine is an antihistamine, but it is also used to increase appetite in cats and dogs, particularly cats undergoing chemotherapy.