In the study, the researchers worked with the drug fentanyl, an opioid sold in strong doses on the street and responsible for an uptick in overdoses. The researchers slightly modified the contents of a fentanyl molecule while keeping its structure intact so that the immune system would recognize it. Over the course of six weeks, mice were injected with these molecules three times, like vaccine booster shots to train the immune system. At the end of that period, the researchers found that it was impossible to get the mice high, no matter how much fentanyl the mice were given. When the researchers sampled some of the mice's blood and combined it with fentanyl molecules in a petri dish, the antibodies killed the drug molecules, leading the researchers to believe that they would stop the drug in the bloodstream and before they could reach the brain. A month later, most of the vaccinated mice survived a lethal dose of the drug.