Introduction: Although several guidelines recommend that prolonged administration of vancomycin should be preferably carried out by a central venous access, vancomycin is often given peripherally. Whether such risk may be affected by different modalities of administration in terms of dilution and time of infusion, it is a matter of controversy. Methods: This single-center prospective study enrolled all consecutive patients requiring prolonged intravenous infusion of vancomycin (1 g/day) using long peripheral cannula “mini-midline.” Patients were randomized in study group (4 mg/ml) and control group (20 mg/ml). All patients were systematically evaluated every 24 h by the Visual Exit-Site Score and a daily ultrasound scan of the veins of the arm. Results: The daily ultrasound evaluation showed venous thrombosis at the distal tip of the cannula in all patients, in both groups. After this finding in the first 14 patients, the study was interrupted. All thromboses were completely asymptomatic and occurred in absence of any sign of catheter malfunction. The onset of thrombosis was significantly earlier in the control group (ranging from 24 to 48 h) than in the study group (ranging from 48 to 96 h), with an average of 30 ± 11 versus 68 ± 16 h ( p < 0.001). Conclusion: Continuous intravenous infusion of vancomycin should be preferably delivered by a central venous access, as largely recommended by current guidelines, since peripheral infusion is inevitably associated with venous thrombosis, independently from the type of peripheral venous access device adopted (short peripheral cannula vs long peripheral cannula) and from the extent of dilution.