A clear-cut reduction in axial length growth in Asian children using OKL compared to single-vision spectacles (SVS) has been proofed in several studies. In these studies, which are very heterogeneous regarding design, reductions of 36-46% in axial length growth were found compared to a matched control group. Two European prospective cohort studies have investigated the effect of OKL compared to SVS in Spanish children. A reduction in axial growth of 32 % and 38% respectively was found. Side effects of OKL have been evaluated; in a meta-analysis from 2015, including 8 Asian and one of the European studies. Odds Ratio for side effects were 8.87 for the OK group compared to the control group, however all side effects were mild and disappeared after termination of the treatment.