\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{6}{lccccc}}
\toprule
 &  & \multicolumn{4}{c}{\textbf{dose}} \\
\cmidrule{3-6}
 & \textbf{Overall} & 1 mg & 5 mg & 10 mg & 50 mg \\
\midrule
\textbf{value} &  &  &  &  &  \\
\hspace{12.0pt}Mean (SD) & 3 (1.41) & 2.67 (2.08) & 2.67 (1.53) & 4.33 (0.577) & 2.33 (0.577) \\
\hspace{12.0pt}Median [Min, Max] & 3 [1, 5] & 2 [1, 5] & 3 [1, 4] & 4 [4, 5] & 2 [2, 3] \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}