The best database with search engine is PubMed. It is free and the keyword search with PubMed offers optimal update frequency and includes online early articles. It is also quick and easy to use.
Google Scholar is also free but it offers results of inconsistent accuracy. It can help in the retrieval of even the most obscure information but its use is marred by inadequate, less often updated, citation information. It updates once a month. Much information about its content coverage remains unknown and results with Google Scholar are displayed in relation to times of visits from users, not in relation to another index of quality of the publication.
Due to the unprecedented growth of electronic resource (e-resource) availability, one of the questions currently being explored is, “how often are e-resources being cited in my field?”. It uses citations in scholarly works to establish links to other works or other researchers. It is one of the most widely used methods of bibliometrics. Automated citation analysis has changed the nature of the research allowing millions of citations to be analyzed for large scale patterns.
For some this is an important feature, mostly for scientists struggling to survive in an academic swamp. For physicians and patients this feature is not important.
Scopus is the only European database, and both Scopus and Web of Science belong to commercial providers and require an access fee. Both Scopus and Web of science are readily updated for printed literature but do not include online early versions as PubMed does. Both databases include only published articles and not online early ones.
PubMed focuses mainly on medicine and biomedical sciences, whereas Scopus, Web of Science, and Google Scholar cover most scientific fields. Web of Science covers the oldest publications, because its indexed and archived records go back to 1900. PubMed allows the larger number of keywords per search but is the only database of the four that does not provide citation analysis. Scopus includes articles published from 1966 on, but information regarding citation analysis is available only for articles published after 1996.
From a previous post: Google is significantly more efficient in searching for specific medical information than other search engines such as Yahoo!, Ask.com and Wikipedia. Moreover, general search engines guide users to websites likely to contain information on a topic of interest with greater efficiency than directly accessing an individual website (eg, Wikipedia, MDconsult and others). But google was only very efficient in this study probably because it was used by web savvy professionals and the type of information for which they were seeking was very specific.
Using Search Engines to Find Online Medical Information
If you want to use more general search engines such as google instead of these previous discussed biomedical databases there is a good introduction on how to use Google for searching for medical information on PLoS Medicine: Using Search Engines to Find Online Medical Information, freely available and open access.
M. E. Falagas, E. I. Pitsouni, G. A. Malietzis, G. Pappas (2007). Comparison of PubMed, Scopus, Web of Science, and Google Scholar: strengths and weaknesses The FASEB Journal, 22 (2), 338-342 DOI: 10.1096/fj.07-9492LSF
Mohammad Al-Ubaydli (2005). Using Search Engines to Find Online Medical Information PLoS Medicine, 2 (9) DOI: 10.1371/journal.pmed.0020228
P JOHNSON, J CHEN, J ENG, M MAKARY, E FISHMAN (2008). A Comparison of World Wide Web Resources for Identifying Medical Information Academic Radiology, 15 (9), 1165-1172 DOI: 10.1016/j.acra.2008.02.010