Lofentanil is a pure mu-opioid receptor agonist derived from fentanyl. It is the most potent opioid to be administered to humans, about 500-1000 times more potent than morphine. Lofentanil provides a higher affinity quotient with longer dissociation times for the mu-receptors than fentanyl. The clinical study of the compound is difficult because there is a very individual sensibility. The appropriate doses are not easy to evaluate. Reversal of the loventanil depression needs very high and repeated naloxone dose. Practical use of lofentanil is limited. Lofentanil side effects are: nausea, vomiting and sedation.