Da wir nejm jetzt auf so merkwürdige Weise laden, müssen wir einige Optionen erst danach ausführen, damit sie nicht überschrieben werden.
\usepackage[backend=bibtex, style=authoryear, language=english]{biblatex}
\makeatletter
\input{nejm.bbx}
\makeatother
\ExecuteBibliographyOptions{sorting=nyt, minnames=1, maxcitenames=2, maxbibnames=999}\renewcommand*{\finalnamedelim}{\addspace\&\space}
\AtBeginBibliography{\renewcommand*{\finalnamedelim}{\multinamedelim}}\documentclass{article}
\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english,ngerman]{babel}
\usepackage[backend=bibtex, style=authoryear, language=english]{biblatex}
\makeatletter
\input{nejm.bbx}
\makeatother
\ExecuteBibliographyOptions{sorting=nyt, minnames=1, maxcitenames=2, maxbibnames=999}
\renewcommand*{\finalnamedelim}{\addspace\&\space}
\AtBeginBibliography{\renewcommand*{\finalnamedelim}{\multinamedelim}}
\renewcommand*{\entrysetpunct}{\addsemicolon\space}
\setlength{\bibitemsep}{0.8\baselineskip}
\renewcommand*{\bibpagespunct}{\addcolon\space}
\DeclareFieldFormat[article,periodical]{number}{\mkbibparens{#1}}
\renewbibmacro*{volume+number+eid}{%
\printfield{volume}%
%\setunit*{\adddot}%
\printfield{number}%
\setunit{\addcomma\space}%
\printfield{eid}}
\renewbibmacro*{journal+issuetitle}{%
\usebibmacro{journal}%
\setunit*{\addspace}%
\usebibmacro{issue+date}%
\setunit*{\addsemicolon\space}
\iffieldundef{series}
{}
{\newunit
\printfield{series}%
\setunit{\addspace}}%
\usebibmacro{volume+number+eid}%
\setunit{\addcomma\space}%
\usebibmacro{issue}%
\newunit}
\defbibenvironment{bibliography}
{\list
{}
{\setlength{\leftmargin}{\bibhang}%
\setlength{\itemindent}{-\leftmargin}%
\setlength{\itemsep}{\bibitemsep}%
\setlength{\parsep}{\bibparsep}}}
{\endlist}
{\item}
\addbibresource{biblatex-examples.bib}
\begin{document}
\cite{sigfridsson,aksin,baez/article,companion,worman,geer}.
\printbibliography
\end{document}