Traditionally, the choice of which candidate compounds to take forward into development has been based on pre-clinical data. However, lack of predictivity of the human clinical situation in the models used has led to poor decision-making, and the later in the development process that such mistakes are realised, the more costly and time-consuming it is to correct them. Furthermore, compounds that may have made perfectly good drugs, have been dropped due to poor pharmacokinetics; in animal models. Accelerator mass spectrometry (AMS) is an ultra-sensitive detection technique that can be used to quantify carbon-14. By administering very small amounts of (14)C-labelled compounds, AMS can be used to obtain human clinical data very early in the drug development process. Such studies: a) can be helpful in understanding human pharmacokinetics using microdosing; b) can provide early human metabolism information, to validate the choice of animal species used in pre-clinical safety testing and identify unique or disproportionate human metabolites during Phase 1; and c) can provide fundamental human pharmacokinetic data, including absolute bioavailability, by facilitating a scientifically optimal and cost-effective study design. The provision of these clinical insights at the earliest possible opportunity can lead to improved decision-making, and therefore can reduce the time and cost involved in the drug development process.