Private pharmacies are the first line of health care in many communities, commonly selling antibiotics in small doses and prescription-only drugs such as steroids without medical supervision. The aim was to study the effectiveness of a multi-faceted intervention on the dispensing practices of drug sellers in Hanoi and Bangkok. The study was a randomized, controlled trial with 68 Hanoi and 78 Bangkok pharmacies, randomly selected and assigned for intervention and control. Behaviour was assessed by five simulated client visits per pharmacy per dispensing practice, at baseline and a month or more after each intervention. Three three-month interventions were implemented sequentially with four months in between: enforcement of regulations with local inspectors visiting to emphasize the importance of prescription-only medicine legislation; education, performed face-to-face in Hanoi and by a large group in Bangkok; and peer review, voluntary in Bangkok and compulsory in Hanoi. The intervention resulted in significant improvements in Hanoi, reducing the dispensing of illegal steroids (29% vs. 62%) and low dose antibiotics (69% vs. 90%), sustained by means of the peer review (17% vs. 57% steroids and 71% vs. 95% antibiotics), and in fewer dispensers asking no questions and giving no advice (11% vs. 30% steroids and 51% vs. 81% antibiotics). The only significant improvement in Bangkok was the reduction in illegally dispensing steroids (25% vs. 44%) after the regulatory intervention. In Bangkok, fewer of those in the group who volunteered for the peer review asked no questions and gave no advice for low-dose antibiotics requests after the peer review (58% vs. 81%). A multi-component intervention can have a profound effect in changing dispensers' behaviour, but the effect is dependant on the context and the method of implementation. Possible reasons for differences are discussed.