In 1992, Stevens worked with researchers at Gilead on the structural studies of neuraminidase inhibitors that eventually became Tamiflu, and later partnered with Roche. After the initial experience with structure based drug discovery from 1992 to 1997 with Gilead and Tamiflu, Stevens focused on understanding the basic mechanism of how Botox (botulinum toxin) works, and on ways to use this scaffold for next generation protein therapeutics. In parallel to the work on botulinum toxin, he worked on the enzymes involved in the catecholamine biosynthetic pathway, specifically the three aromatic amino acid hydroxylases including phenylalanine hydroxylase. From 1999 to 2004, Stevens was involved in the startup of Syrrx that developed the marketed drug Nesina for type II diabetes. From 2000 to 2010, Stevens has worked with BioMarin Pharmaceutical to develop Kuvan (tetrahydrobiopterin) and assisted in the design and development of PEG-PAL (pegylated Phenylalanine ammonia-lyase) as treatments for mild and classical phenylketonuria (PKU).  In 2008, Stevens started Receptos that developed an S1P1 agonist for multiple sclerosis and inflammatory bowel disease, now on the market called Zeposia and sold by BMS.