documentclass[12pt]{article}

RequirePackage[most]{tcolorbox}

newtcolorbox[auto counter]{Boxone}[1][]{%
enhanced,breakable,
before title={stepcounter{BoxOnecnt}},
toggle left and right,sharp corners,
boxrule=0mm,top=0mm,bottom=0mm,left=0mm,right=0mm,
colframe=white,
colback=black!30,
fonttitle=bfseries,
left=0pt,%%21.5
leftrule=21.5pt,
rightrule=0pt,
width=hsize,
overlay unbroken and first ={%
node[rotate=90,
minimum width=21.5pt,
anchor=south,
font=fontsize{11.5bp}{11.5bp}sffamilybfseries,
yshift=2.5pt,
white]
at (frame.east) {WORKED E.G.~1};
},
}


begin{document}


begin{Boxone}%[WORKED E.G. 1.1]
subsection{Addition}
A patient asks you how many medicines they need to swallow to take all their morning medicines. They have the following medicines to take:
begin{itemize}
item 500 mg paracetamol (2 capsules)
item 40 mg frusemide (1 tablet)
item 62.5 mcg digoxin (2 tablets)
end{itemize}
A patient asks you how many medicines they need to swallow to take all their morning medicines. They have the following medicines to take:
end{Boxone}

end{document}