In this systematic review, we evaluated studies involving adjuvant and primary topical treatment for ocular surface squamous neoplasia (OSSN). The findings were: (i) adjuvant 5-fluorouracil (5-FU) reduces the risk of relapse after surgical excision with mild side effects [level Ib, grade of recommendation (GR) A]. (ii) Primary topical mitomycin (MMC) produces a high rate of complete response, low recurrence rate, and mild side effects (level Ib, GR A). (iii) Primary chemotherapy versus adjuvant chemotherapy produce similar rates of recurrence, with no significant difference (level IIb, GR B). (iv) Adjuvant 5-FU versus MMC showed no significant differences, with mild side effects in both groups and a better toxicity profile for MMC (level III, GR C). (v) Primary topical 5-FU versus MMC versus interferon (IFN) showed similar rates of tumor recurrence, mild side effects for all drugs, and more severe side effects in the 5-FU arm, followed successively by MMC and IFN (level III, GR C).