Injected intramuscularly, the enanthate and decanoate esters of the phenothiazine fluphenazine are an effective treatment of the disordered behavior and thinking of schizophrenia. The decanoate preparation is not only slightly longer-acting but also has a smaller incidence of side-effects than the enanthate. The major adverse effect of these medications is the high frequency of extrapyramidal system disturbance.
Since the 50% rate of failure of schizophrenic outpatients to take prescribed oral medications decreases treatment failure to about 20% with the use of long-acting injectable phenothiazines, this route of administration offers an advantage in patient management particularly applicable to community mental health systems. Moreover, parenteral administration of long-acting fluphenazine may be useful for patients who do not attain effective serum levels with medication taken orally because of metabolic or absorption difficulties.
James E. Groves, Michel R. Mandel. The Long-Acting Phenothiazines. Arch Gen Psychiatry. 1975;32(7):893–900. doi:10.1001/archpsyc.1975.01760250085009