Eliodoro Donato Mercado (1866-1933) was an expert on leprosy in the Philippines. He devised a method of injecting chaulmoogra oil in 1914 that was adopted as the Mercado method. He was the resident physician in the leprosy department of San Lazaro Hospital, Manila and a member and founder of the Colegio Médico-Farmacéutico of the Philippines and an ex-supernumerary surgeon in the Spanish Army.

His publication on the Mercado method is Leprosy in the Philippines and its Treatment. https://archive.org/stream/adr1042.0001.001.umich.edu#page/n3/mode/2up

In "Chaulmoogra Oil and the Treatment of Leprosy", John Parascandola writes about Mercado's role in the development of chaulmoogra oil treatment:

[Victor] Heiser was a career Public Health Service Officer who was serving as the Chief Quarantine Officer and Director of Health for the Philppine Island at the time of this work. Chaulmoogra oil had been used at the San Lazaro Hospital for leprosy in Manila since the early years of the American occupation of the Philippines, but with limited success. In 1908, Heiser visited the Louisiana Leprosy Home and gained a favorable impression of the treatment of leprosy with Chaulmoogra oil that was being carried out there. From Ralph Hopkins and Isadore Dyer, Heiser learned better techniques for the oral administration of the oil. Upon his return to the Philippines, he arranged for the Louisiana method to be given a thorough trial at the San Lazaro Hospital in Manila under the immediate charge of the house physician, Elidoro Mercado.

The new method, which was initiated at San Lazaro in 1909, was much more successful than the former one, but still oral administration of the oil resulted in nausea and patient resistance to taking the drug. The physicians tried various methods to solve this problem, such as coating the Chaulmoogra capsules with various substances and giving the oil by enema, but the results were unsuccessful. A review of the literature revealed that some physicians had tried hypodermic injection. So the hypodermic method was tried at San Lazaro, but it did not work well because the oil was not satisfactorily absorbed. Heiser then wrote to Merck and Company in Germany to ask if they could suggest a substance that might increase the absorption of the oil when injected. The company replied that it had no definite knowledge on the subject, but that theoretically the addition of camphor or ether might have the desired result.

Heiser and Mercado decided to add camphor to a prescription of Chaulmoogra and resorcin which was typically given orally. To their great joy they found that the camphor-resorcin solution of Chaulmoogra was readily absorbed. Heiser wrote in his autobiography years later about the excitement of the first case treated: “Few can imagine with what a thrill we watched the first case to which chaulmoogra was administered in hypodermic form, how we watched for the first faint suspicion of eyebrows beginning to grow in again and sensation returning to paralyzed areas. We took photographs at frequent and regular intervals to compare progress and to check on our observations, fearing our imagination might be playing tricks upon us...” (pp. 9-10)

Sources:

John Parascandola, “Chaulmoogra Oil and the Treatment of Leprosy” http://www.lhncbc.nlm.nih.gov/files/archive/pub2003048.pdf