Developed by a team led by the University of Houston, the vaccine incorporates a fentanyl-like hapten.

A hapten is a small molecule which, when combined with a larger carrier protein, elicits the production of antibodies which bind to it. In the case of this vaccine, that protein is in fact a genetically deactivated diphtheria toxin known as CRM197 – it is already used in multiple FDA-approved vaccines.

Once the antibodies have been produced, they bind not only to the fentanyl-like hapten molecules, but also to any actual fentanyl molecules present in the bloodstream. Those molecules are thus unable to enter the brain, so they don't produce any feelings of euphoria. They ultimately end up being passed from the body via the kidneys.