Cite all the elements as for a print article, but in addition:
Authors(s). (Surname followed by initial(s)).
Article title.
Journal name [Internet].
Year of publication [cited date Year Month Day];
Volume number
(Issue number):
Inclusive page number(s) of the article. (If available - if not count or estimate, e.g. [6 p.] or [ about 12 p.] )
Available from:
Example:
Wolinsky FD, Bentler SE, Hockenberry J, Jones MP, Weigel PA, Kaskie B, et al. A prospective cohort study of long-term cognitive changes in older Medicare beneficiaries. BMC Public Health [Internet]. 2011 [cited 2012 Jan 1];11(710): 357-69. Available from: http://www.biomedcentral.com/1471-2458/11/710
Jafri H, Alsheikh-Ali AA, Karas RH. Meta-analysis: statin therapy does not alter the association between low levels of high-density lipoprotein cholesterol and increased cardiovascular risk. Annals of Internal Medicine [Internet]. 2010 [cited 2013 Jun 12];153(12): [1 p.]. Available from: http://annals.org/article.aspx?articleid=746580
Rothwell PM. Factors that can affect the external validity of randomised controlled trials. PLoS Clinical Trials [Internet]. 2006 [cited 2012 Jun 21];1(1): e9 [5 p.]. Available from: doi: 10.1371/journal.pctr.0010009