Objective: PubMed is one of the most commonly used search tools in biomedical and life sciences. Existing studies of database coverage generally conclude that searching PubMed may not be sufficient although some find that the contributions from other databases are modest at best. However, generalizability of the studies of the coverage of PubMed is typically restricted. The objective of this study is to analyze the coverage of PubMed across specialties and over time. Study Design and Setting: We use the more than 50,000 included studies in all Cochrane reviews published from 2012 to 2016 as our population and examine if the studies and resulting publications can be identified in PubMed. Results: The results show that PubMed has a coverage of 70.9, 95% confidence interval (CI) (68.40, 73.30) of all the included publications and 82.8%, 95% CI (80.9, 84.7) of the included studies. There are huge differences in coverage across and within specialties. In addition, coverage varies within groups over time. Conclusion: Databases used for searching topics within the groups with highly varying or low coverage should be chosen with care as PubMed may have a relatively low coverage.