Mitragynine is the most abundant alkaloid component of the psychoactive plant material “kratom”, which according to numerous anecdotal reports shows efficacy in self-medication for pain syndromes, depression, anxiety, and substance use disorders. We developed a new synthetic method for selective functionalization of the unexplored C11 position of the mitragynine scaffold (C6 position in indole numbering) via the use of an indole-ethylene glycol adduct and subsequent iridium-catalyzed borylation. We discovered that C11 represents a key locant for fine-tuning opioid receptor signaling efficacy. In the 7-hydroxymitragynine (7OH) series, the high efficacy parent compound was transformed to an equipotent low efficacy agonist by introducing a fluorine substituent in this position (11-F-7OH), as demonstrated in vitro at both mouse and human mu opioid receptors (mMOR/hMOR) and in vivo in mouse analgesia tests after systemic administration. Low efficacy opioid agonists are of high interest as candidates for generating safer opioid medications with mitigated adverse effects.