One problem is that when sanctions were tightened in 2012, Iran’s ability to sell oil was further limited and its main source of hard currency restricted. Another problem is that Iran’s main banking infrastructure — including the Central Bank of Iran and Bank Tejarat, Iran’s main trading bank — is blacklisted by Washington.

Sanctions have also choked-off Iranian banks from the global financial arena by putting draconian restrictions on international banks that deal with Iran, including by cutting them off from the Society for Worldwide Interbank Financial Telecommunication. Penalties for violating U.S. sanctions are so stern as to discourage most international banks, which are generally risk-averse anyway, from engaging in humanitarian trade.

A senior representative from a reputable Iranian pharmaceutical company told our study group that when he presented a French bank in Paris with documentation showing that a deal to ship vaccines to Iran was fully legal, he was told, “Even if you bring a letter from the French president himself saying it is O.K. to do so, we will not risk this.” Today, only one international bank — in Turkey — is willing to take the chance.

In simple terms, even when Iran can get its hands on dollars or euros to buy medical supplies, it cannot find a banking avenue to clear the trade. A senior representative at one American pharmaceutical company told me about a $60 million order for an anti-rejection drug for liver-transplant patients that fell through even though the sale was fully legal, all the needed licensing from the U.S. Treasury was in place, and Iran had allocated the needed hard currency. No bank would perform the transaction.

To compensate, Iran has been importing more drugs, or the active ingredients for them, from China and India. But these products are usually of inferior quality and more limited effectiveness than the equivalent from American and European manufacturers. And in the highly patented world of pharmaceuticals, substitution often isn’t an option at all, particularly when it comes to advanced medicines used to treat complex diseases like cancer and multiple sclerosis.