In my opinion, it is better for you to use the internal standard (IS) for analyzing drug concentration in pharmacokinetic study because there are some extraction procedures. You will need the internal standard because it can be a correction factor when you calculate the concentration, so you will not calculate the area, but you will calculate the peak area ratio (PAR) between the drug and IS. PAR is used for making calibration curve.
Calibration curve is required for both the external standard and internal standard procedure. Please see the link below which explain you in brief with example.