In Ecuador, as in most Third World nations, thousands of different prescription-only pharmaceuticals can be bought without a doctor's prescription. But how often does self medication actually occur? This study documents 619 prescription drug sales in two Ecuadorian pharmacies. In 51% of these sales, customers in fact present no prescription. Many of the drugs sold this way have serious side effects and must be used with care. Ecuadorian law greatly restricts information on drug packaging about side effects, indications, contraindications, schedule and dosage. Although the pharmacies differ with respect to self medication rates, drug choices, and clerk-customer interactions, both show the existence of a 'shadow system of biomedicine' in which prescription drugs are used without physician consultation. In view of the dominant role that transnational corporations play in Third World pharmaceuticals usage, this analysis incorporates a political economic perspective.