Amitriptyline hydrochloride - originally sold under brand names Elavil, Endep and others, is a second-line drug used to treat Major Depressive Disorderplugin-autotooltip__plain plugin-autotooltip_bigMajor Depressive Disorder

Major Depressive Disorder (MDD), also known simply as 'depression', is a mental disorder characterized by at least two weeks of pervasive low mood.

Current estimates are that severe depression affects around 2% of the global population at any given time, currently more than 150 million. The majority of cases don't appear to have an obvious personal and/or societal cause., Migraineplugin-autotooltip__plain plugin-autotooltip_bigMigraine

"Chronic migraine has a great detrimental influence on a patient’s life, with a severe impact on socioeconomic functioning and quality of life. Chronic migraine affects 1–2% of the general population, and about 8% of patients with migraine; it usually develops from episodic migraine at an annual conversion rate of about 3%.", Parkinson's diseaseplugin-autotooltip__plain plugin-autotooltip_bigParkinson's disease

Also known as Parkinson disease, Parkinson's, idiopathic Parkinsonism, primary Parkinsonism, PD, or paralysis agitans - is a degenerative disorder of the central nervous system resulting from the death of dopamine-containing cells in the and, recently, Attention-Deficit / Hyperactivity Disorder (ADHD)plugin-autotooltip__plain plugin-autotooltip_bigAttention-Deficit / Hyperactivity Disorder (ADHD)

Attention Deficit Hyperactivity Disorder (ADHD) is a neuro-developmental mental disorder. It's characterized by problems in paying attention, excessive activity, or difficulty in controlling behavior (typically not appropriate for a person's age). Symptoms usually begin at between six to twelve years of age, and, to be classified as ADHD must be present for more than six months and cause problems in at least two settings (e.g. school, home, or i…

It was discovered more than 70 years ago, and, despite a wide range of debilitating side effects, and the discontinuation of Elavil and Endep, it's still in use. In the US alone there are around 8 million prescriptions per year.

Amitriptyline hydrochloride is an antidepressant with sedative effects. Its mechanism of action in man is not known. It is not a monoamine oxidase inhibitor and it does not act primarily by stimulation of the central nervous system.
Amitriptyline inhibits the membrane pump mechanism responsible for uptake of norepinephrine and serotonin in adrenergic and serotonergic neurons. Pharmacologically this action may potentiate or prolong neuronal activity since reuptake of these biogenic amines is important physiologically in terminating transmitting activity. This interference with reuptake of norepinephrine and/or serotonin is believed by some to underlie the antidepressant activity of amitriptyline.

Source : US NIH

Note : For some uses, e.g. in treating ADHD and Parkinson's, there is currently no formal agreement about whether it's effective or not.