\begin{tikzpicture}
	\begin{pgfonlayer}{nodelayer}
		\node [style=Red Box] (0) at (4, -1.5) {Will Terminate?};
		\node [style=Red Box] (1) at (-4.25, -1.5) {Alternate Drugs};
		\node [style=purple box] (5) at (0, 2) {Enrollment};
		\node [style=Gray Box] (18) at (-6.5, 1.5) {Population};
	\end{pgfonlayer}
	\begin{pgfonlayer}{edgelayer}
		\draw [style=RightArrow] (18) to (5);
		\draw [style=RightArrow] (18) to (1);
		\draw [style=PurpleArrow] (1) to (5);
		\draw [style=PurpleArrow] (5) to (0);
		\draw [style=PurpleArrow] (1) to (0);
	\end{pgfonlayer}
\end{tikzpicture}