The majority of Americans believe that lowering drug prices should be the top health care priority for the federal government. Yet drug costs as a proportion of the country's medical expenditures have increased substantially in recent years. Because drugs are basic necessities, and because how much society should contribute toward providing basic necessities is a question of justice, policies regarding drug prices must fulfill principles of justice, not just economic efficiency. In this article I define a standard for when the price of a drug is unjust, using a cross-disciplinary ethical approach. Based on four principles, I propose the average lifetime earnings standard for affordability. According to this standard, a drug price is unfair if it exceeds 11 percent of the average American's disposable income. This suggests that current prices for many drugs are excessive and unjust.