Emily,

I think it's a mistake to withhold information about a particularly dangerous drug. When you're dealing with uneducated, ignorant people, your obligations to disclosure are greater, not less. One of the first things most of us ask when the MD recommends something is, "What's the downside".

These people don't understand that, and if they start to make a connection between the drugs and inevitable side effects, what do you think their conclusions will be if/when they discover the Gringos knew this all along and did nothing? If they have a child on their own and that child dies a natural death, you're free of blame, but if you come into their village and introduce something which subsequently causes them harm, and you knew it could happen and didn't tell them, how different in their minds will that be from the Conquistadors?