Abstractobjective To evaluate the quality of omeprazole personally imported into Japan via the Internet and to compare the quality of these samples with previously collected samples from two other Asian countries.methods The samples were evaluated by observation, authenticity investigation and pharmacopoeial quality analysis. Quality comparison of some selected samples was carried out by dissolution profiling, Raman spectroscopy and principle component analysis (PCA).results Observation of the Internet sites and samples revealed some discrepancies including the delivery of a wrong sample and the selling of omeprazole without a prescription, although it is a prescription medicine. Among the 28 samples analysed, all passed the identification test, 26 (93%) passed the quantity and content uniformity tests and all passed the dissolution test. Dissolution profiling confirmed that all the personally imported omeprazole samples remained intact in the acid medium. On the other hand, six samples from two of the same manufacturers, previously collected during surveys in Cambodia and Myanmar, frequently showed premature omeprazole release in acid. Raman spectroscopy and PCA showed significant variation between omeprazole formulations in personally imported samples and the samples from Cambodia and Myanmar.conclusions Our results indicate that the pharmaceutical quality of omeprazole purchased through the Internet was sufficient, as determined by pharmacopeial tests. However, omeprazole formulations distributed in different market segments by the same manufacturers were of diverse quality. Measures are needed to ensure consistent quality of products and to prevent entry of substandard products into the legitimate supply chain.