<P> Antidepressants are drugs used for the treatment of major depressive disorder and other conditions, including dysthymia, anxiety disorders, obsessive--compulsive disorder, eating disorders, chronic pain, neuropathic pain and, in some cases, dysmenorrhoea, snoring, migraine, attention - deficit hyperactivity disorder (ADHD), addiction, dependence, and sleep disorders . They may be prescribed alone or in combination with other medications . </P> <P> Some typical side effects of antidepressants are dry mouth, weight gain, and lack of sex drive . Some antidepressants may even cause infertility within both males and females depending on the extent of how long the medication was taken or how the body reacted to the medicine . Most types of antidepressants are typically safe to take, but may cause increased thoughts of suicide when taken by children, adolescents, and young adults . When prescribing antidepressants, doctors will warn individuals not to abruptly stop taking the medication or miss several doses at one time; this could cause major withdrawal - like symptoms, and quitting the medication suddenly can worsen the depression . Patients are instead encouraged to communicate and work with their doctors to wean themselves off the medication by gradually and safely decreasing the dosage when needed . </P> <P> The most important classes of antidepressants are the selective serotonin reuptake inhibitors (SSRIs), serotonin--norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs), reversible inhibitors of monoamine oxidase A (RIMAs), tetracyclic antidepressants (TeCAs), and noradrenergic and specific serotonergic antidepressant (NaSSAs). St John's wort is also used in the treatment of depression . </P> <P> One theory regarding the cause of depression is that it is characterized by an overactive hypothalamic--pituitary--adrenal axis (HPA axis) that resembles the neuro - endocrine response to stress . These HPA axis abnormalities participate in the development of depressive symptoms, and antidepressants may serve to regulate HPA axis function . </P>

What are two of the major classes of antidepressant drugs