Emporio Ivankov's Hormone Therapy is a weird and powerful ability brought on by the Horu Horu no Mi, a Paramecia-like devil fruit that gives its consumer the power to create and control hormones. Ivankov, one of the key series characters, unleashed a great deal of versatility and power with the ability throughout the series—very notably in the Impel Down and Marineford sagas.
The core power of Horu Horu no Mi is that it allows its consumer to produce special hormones, which can drastically alter the human body in a very short time. Ivankov, who learned how to control this fruit, has done many things with it, from combat to medicine. One of the most famous techniques is "Emporio Onna Hormone" (literally, "Emporio Female Hormone"), which gives a man a shot with special female hormones to become a woman. This power reveals how this fruit could change important biological features.
Another major application of Ivankov's hormone therapy is the "Emporio Tension Hormone". It is the technique by which one administers a dosage of adrenaline to an extremely fatigued person, releasing tension and giving easier movement for the course of one day. It may turn out to be quite useful in tight situations and allow allies to work their bodies harder than what could be humanly possible.
Probably, the most dramatic use of Ivankov's hormone therapy is during the Impel Down arc when he is to treat Luffy. He uses his powers of the Devil Fruit to stimulate Luffy's immune system, thereby healing his wounds and counteracting the poison from the severe poisoning by Magellan. Such availing treatment comes at a great cost. It is believed that these hormones have put such a large strain on Luffy's body that his life expectancy has been cut short. What this problem with the therapy brings into focus is the double-edged nature of Ivankov's powers: while they are capable of producing miracles in healing and empowerment, they also hold serious dangers and possible long-term effects.
Ivankov's hormone therapy also has a role in more subtle plot developments. Fans have speculated that maybe Ivankov once used his ability on Crocodile, making him change sex. The proof for this theory is based on Ivankov stating that he knows of a "weakness" Crocodile has, though this was never confirmed. Still, the fact such speculation is possible illustrates the storytelling potential Ivankov's ability holds for creating complicated backstories and relationships between characters.
The powers of Horu Horu no Mi stretch much further than changing one's sex and body enhancement. Ivankov can also use hormones to change facial structure. This is shown, for instance, in his case when he transforms his head into a bigger replica, presumably by changing growth hormones. Such kinds of ability in application enhance unpredictability in Ivankov's fighting styles and support.