Up to eighty percent of all patients admitted to hospital worldwide will receive a peripheral IV and this procedure is now considered indispensable to human health. However, despite its global use, the choice of catheter is not always governed by clear and universal guidelines. After reviewing the few best-practice recommendations which exist, we propose a patient--and therapy--driven matrix for deciding on the gauge and length of peripheral catheter for the individual patient. This matrix takes patient age, clinical stability, current state of veins, therapy duration and the nature of the medication to be delivered into consideration. Use of such a matrix will not deliver a formulaic answer but will orient choices along logical, evidence-based lines. This approach will be an advance on the all-too-common reliance on habit and tradition in the choice of peripheral IV catheter.