Purpose: Antipsychotic efficacy in schizophrenia depends on its availability in the body. Although therapeutic outcomes remain still far from satisfactory, therapeutic drug monitoring not a common part of clinical practice during treatment with long-acting injectable antipsychotics (LAI AP). The real effectiveness of LAI AP is thus uncertain. Patients and Methods: We made a retrospective evaluation of plasma levels of LAI AP. Collection of blood samples was performed just before the drug application and one week later. Forty patients with a stabilized clinical condition and steady-state plasma levels were included. Results: In the observed cohort of patients, flupentixol decanoate (n = 23) was the most often used drug, followed by fluphenazine decanoate (n = 7), haloperidol decanoate (n = 5), paliperidone palmitate (n = 3), and risperidone microspheres (n = 2). Just 5 of 40 patients were treated with a monotherapy. In the period before the application, 60% of the patients did not reach the therapeutic reference range (TRR) and 20% of the patients had an undetectable plasma level. At the time of collection of the second blood samples performed after 7 days, 24% of the patients were under the TRR. Conclusion: We have found a surprisingly high incidence of plasma levels under the TRR patients treated with LAI AP. Notwithstanding individual variability in pharmacokinetics, seems that LAI AP may be underdosed in usual clinical practice.