Methionine is a protein-based amino acid which assists with metabolic function, breaks down fat, and is the primary source of sulfur in the body. As one of the compounds known as a lipotrophic, a fat burner or antioxidant, methionine is also critical for the removal of heavy metals, such as lead and mercury, from the body. Some of its other primary purposes include ensuring healthy liver, kidney, bladder, and artery functions; the maintenance of healthy nails, hair, and skin; and it is essential for muscle growth and energy.

The human body does not naturally produce methionine, therefore it must be ingested in food, via supplemental pill form, or administered intravenously in the case of illness. Side effects of supplements may include nausea, vomiting, and drowsiness. There are no known negative interactions of methionine with other drugs, but mixing any medication should never be done without the authorization from a doctor.

Methionine is found naturally in foods such as red meat, fish, beans, and grains, but can also be produced synthetically as a supplement. Pregnant women or nursing mothers should not take this amino acid in supplement form without consulting with their doctor. Women who take oral contraceptives can take methionine; as it increases the production of estrogen, they should talk with their doctor beforehand.

Substances, such as histamine, which can cause allergic reactions and dilate blood vessels can affect the way the brain sends and receives messages. Methionine works to reduce histamine levels in the body to allow proper synaptic function. Deficiencies in methionine levels can lead to severe mental disorders such as dementia, and in supplement form it is often prescribed for the treatment of schizophrenia. It has also been shown as a promising agent to assist with memory recall and the treatment of other mental disorders such as Parkinson's and Alzheimer's Disease, and for patients suffering from HIV/AIDS, attention deficit hyperactivity disorder (ADHD) in adults, fibromyalgia, and stress and anxiety.

Methionine is increasingly prescribed by alternative medical practitioners to boost protein levels for vegetarians and vegans who may not receive enough strictly through fruits, vegetables, and grains. It is also beneficial for those who metabolize large quantities of proteins, such as athletes. This amino acid is also often recommended as a protein replacement for people who are considered heavy drinkers.

Some common and significant side effects of methionine deficiency include liver damage, edema, weakness, and brittle hair. Low levels can slow normal growth and development in children. Insufficient levels in pregnant women may result in neural tube defects in infants, which are brain and spinal column disorders such as hydroencephaly or spina bifida.