The major psychiatric drugs fall into three categories: Corrective, symptomatic and counter-pathological.
Corrective drugs (when they work) tend to correct the underlying pathology; symptomatic drugs (when they work) do not affect the underlying pathology but relieve troublesome symptoms; while counter-pathological drugs create an alternative pathology that (in some way) tends to counter troublesome symptoms.
An example are the psychostimulant drugs, such as amphetamines or methylphenidate ('Ritalin'). These are drugs which increase central dopaminergic activity - and are generally used in people where dopaminergic activity is low or deficient.
Some tranquillizers such as benzodiazepines (or the earlier 'Miltown'/ meprobamate) act to diminish anxiety states, probably by damping-down the same brain systems which cause anxiety - and could perhaps be regarded as corrective.
I would place electroshock/ electroconvulsive therapy (ECT) in the corrective category. It probably works partly by normalizing the coordination of the brain (as seen in the 'brainwaves' of the EEG) and breaking a
positive feedback loop of sleep disturbance leading to behavioural
change which perpetuates sleep disturbance - as happens in ECT treatment of severe
melancholia with psychosis, in mania and acute schizophrenia. The
therapeutic effect of ECT on, for example Parkinson's disease and
Catatonia, also suggests that dopaminergic deficiency states are
Tricyclic antidepressants such as Imipramine and Amitriptyline can be used to treat moderately severe 'endogenous' depression, where they probably act as analgesics/ pain killers to treat symptoms of malaise (fatigue, aches and pains, 'feeling ill'), treat insomnia and reduced appetite/ weight loss.
Hypnotics, or sleeping drugs (strong sedatives) are used to treat insomnia, and also the psychotic results of sleep disturbance and deprivation.
Neuroleptic/ antipsychotic drugs cause symptoms of Parkinson's disease, and this pathology may counteract the symptoms of psychotic illness such as agitation - the demotivation which comes with Parkinsonism tends to make people docile, stops them listening to hallucinatory voices, stops them acting upon delusional ideas.
Lithium also produces the counter-pathology of emotional blunting - and this can be used to treat mania; and to prevent manic or depressive breakdowns.
SSRIs when used to treat anxiety and mild depression produce a milder version of the demotivation and emotional blunting of neuroleptics (because SSRIs are chemically related to neuroleptics, being derived from the same antihistamines as are neuroleptics). Thus people with emotional instability, anxiety, panic, phobias, shyness etc. - may have these symptoms implores by the counter-pathology of emotional blunting.
There is a hierarchy implicit in this classification system: the most effective treatments (potentially) are corrective (although they may have other problems, such as addiction); while the worst treatments are counter-pathological - since even when effective on the target problems these will always have significant 'side effects' because the core 'side effects' are in fact the counter-pathology caused by treatment.