a significant indicator of the link between a drug and an adverse event

To introduce a drug again after an initial adverse result. For example, if someone has an allergic reaction to a drug, he or she may be rechallenged by taking a smaller dose and building up to a normal dose.

to administer the same substance (e.g., pathogenic organism, drug) a second or subsequent time. Rechallenge may be done after vaccination to determine whether a vaccine was effective.