Bottom line:  Properply used barcode medication administration technology reduces the rate of medication errors in acute care settings.

Summary:  San TH, et al.  Factors affecting registered nurses’ use of medication administration technology in acute care settings: A systematic review.  JBI Library of Systematic Reviews. 10(8):471-512,  2012.
Although this systematic review addresses nurses’ attitudes and barries to using technology like barcode scanning, the Background section (p. 474), documents evidence on effectiveness of using barcodes in reducing medication errors.

Cited studies
Poon EG, Keohane CA, Yoon CS, Ditmore M, Bane A, Levtzion-Korach O, et al. Effect of bar-code technology on the safety of medication administration. New Engl J Med. 2010;362(18):1698-707.
Before and after study at academic medical center implementing barcode system. RESULTS: Investigators observed 14,041 medication administrations and reviewed 3082 order transcriptions. Observers noted 776 nontiming errors in medication administration on units that did not use the bar-code eMAR (an 11.5% error rate) versus 495 such errors on units that did use it (a 6.8% error rate). 41% reduction in error rate after the barcode system was implemented.

Morriss FH, Abramowitz PW, Nelson SP, Milavetz G, Michael SL, Gordon SN, et al. Effectiveness of a barcode medication administration system in reducing preventable adverse drug events in a neonatal intensive care unit: a prospective cohort study. Journal Ped. 2009;154(3):363 – 8.
Observational study conducted during the process of installing a barcode medication administration system in a neonatal ICU. RESULTS: 92 398 medication doses were administered to 958 subjects. Relative risk of medication errors when using the barcode system was 0.53 compared to not using the barcode system.

Other studies

See this PubMed search for a focus on quasi-experimental studies:
“Medication Systems, Hospital”[MAJR] AND “Medication Errors/prevention and control”[MAJR] AND (barcodes OR barcoding OR bar codes) AND (comparative OR before and after OR quasi-experimental)

To view a broader set of references on this topic:
“Medication Systems, Hospital”[MAJR] AND “Medication Errors/prevention and control”[MAJR] AND (barcodes OR barcoding OR bar codes)