Siddiquis first breakthrough in research came when he successfully isolated an antiarrhythmic agent in 1931 from the roots of Rauvolfia serpentina. He named the newly discovered chemical compound as Ajmaline, after his mentor Hakim Ajmal Khan who was one of the illustrious practitioners of Unani system of medicine in South Asia. Later on, Siddiqui also extracted other alkaloids from Rauvolfia serpentina that included Ajmalinine, Ajmalicine (CHNO), Isoajmaline, Neoajmaline, Serpentine and Serpentinine'. Many of these are still used worldwide for treatment of mental disorders and cardiovascular ailments, especially as antiarrhythmic agents in Brugada syndrome.