I just had an l3-s1 fusion. do i need to be taking anti rejection medication?

No you don't: You don't need any anti-rejection medication. Your immune system does not form antibodies to implants. Even though at times rarely one may be allergic to the metal used, and may have allograft (cadaver) bone used in the fusion, it still does not typically trigger any significant immune response or rejection.

No: After spinal fusion, i would recommend you avoid nsaids or steroids. These inhibit inflammation which actually helps the fusion process to complete. Otherwise, meds as prescribed by your surgeon and resumption of other medication prescribed by other doctors is fine (even blood thinners generally ok a week or so after fusion). Rejection meds are not typically used after spine fusion.