Aripiprazole as a long-acting injectable (LAI) is initiated in oral aripiprazole-stabilised patients and needs, after first injection, 14 days supplementation of oral aripiprazole (one-injection start, OIS). Recently, an alternative two-injection start (TIS) was advanced, involving two 400 mg injections with a single 20 mg oral supplementation of aripiprazole. We tested the two regimens in patients with schizophrenia (SCZ, n = 152, 90 men and 62 women) with (SUD⁺; n = 93) or without (SUD^–; n = 59) substance use disorders (SUDs), comparing OIS (n = 66) with TIS (n = 86) and SUD⁺ vs. SUD^–. For 26 patients, we measured weekly for one month, aripiprazole + dehydroaripiprazole (active moiety) levels. Patients were followed for three months after LAI with psychopathology and quality-of-life scales (BPRS, CGI-S, ACES, BIS-11, and WHOQOL). All groups improved in psychopathology with no differences between OSI and TIS and between SCZ–SUD⁺ and SCZ–SUD^–. The TIS group was associated with serum blood levels of the active moiety within the therapeutic window, while the OIS group showed peaks above the window, possibly exposing patients to toxicity. Treatments were well-tolerated. Here we showed no disadvantages for TIS vs. OIS and possibly increased safety. Shifting the initiation of aripiprazole LAIs to the TIS modality may be safe and pharmacokinetically advantageous.