Chronic opioid consumption is associated with addiction, physical dependence, and tolerance. Tolerance results in dose escalation to maintain the desired opioid effect. Intake of high‐dose or potent opioids may cause life‐threatening respiratory depression, an effect that may be reduced by tolerance. We performed a pharmacokinetic‐pharmacodynamic analysis of the respiratory effects of fentanyl in chronic opioid users and opioid‐naïve subjects to quantify tolerance to respiratory depression. Fourteen opioid‐naïve individuals and eight chronic opioid users received escalating doses of intravenous fentanyl (opioid‐naïve subjects: 75–350 µg/70 kg; chronic users: 250–700 µg/70 kg). Isohypercapnic ventilation was measured and the fentanyl plasma concentration‐ventilation data were analyzed using nonlinear mixed‐effects modeling. Apneic events occurred in opioid‐naïve subjects after a cumulative fentanyl dose (per 70 kg) of 225 (n = 3) and 475 µg (n = 6), and in 7 chronic opioid users after a cumulative dose of 600 (n = 2), 1,100 (n = 2), and 1,800 µg (n = 3). The time course of fentanyl’s respiratory depressant effect was characterized using a biophase equilibration model in combination with an inhibitory maximum effect (E_max) model. Differences in tolerance between populations were successfully modeled. The effect‐site concentration causing 50% ventilatory depression, was 0.42 ± 0.07 ng/mL in opioid‐naïve subjects and 1.82 ± 0.39 ng/mL in chronic opioid users, indicative of a 4.3‐fold sensitivity difference. Despite higher tolerance to fentanyl‐induced respiratory depression, apnea still occurred in the opioid‐tolerant population indicative of the potential danger of high‐dose opioids in causing life‐threatening respiratory depression in all individuals, opioid‐naïve and opioid‐tolerant.