Aims Facial nerve palsy (FNP) is a recognised sequela of head and neck cancer (HNC) surgery, with profound implications on quality of life. Although botulinum toxin (BTx) is increasingly used in FNP rehabilitation, whether its benefit reflects synkinesis suppression or generalised motor recovery, and when the benefit of further injections plateau remains undefined. We quantified the independent effect of BTx treatment on patient-reported outcomes and characterised its dose-response relationship to identify a potential therapeutic plateau. Methodology A retrospective cohort study was conducted with 250 patients at a tertiary facial nerve clinic (February 2015–June 2025). Patients received BTx with physiotherapy (BTx+PT, n=122) or physiotherapy alone (PT, n=128). Outcomes were the Facial Disability Index (FDI) and Synkinesis Assessment Questionnaire (SAQ). Results BTx+PT significantly improved SAQ (−9.86%, p=0.011), FDI (+19.17%; p<0.001), FDI-P (+6.74%; p=0.015), and FDI-S (+12.43%; p<0.001). PT alone significantly improved FDI (+22.17%; p=0.014) and FDI-S (+13.56%; p=0.011). BTx+PT led to a 21.6% greater improvement in SAQ compared to PT alone (p=0.001), with no significant difference in FDI, FDI-S, or FDI-P. Cumulative BTx dose predicted SAQ improvement more strongly than session count (Pearsons r=−0.476 vs −0.400; p=0.002), with each session conferring a −1.53% SAQ reduction (p=0.010). Each additional session was associated with a 1.53% reduction in SAQ (95% CI -2.69 to -0.38, p=0.010), with benefit persisting up to 12 sessions. Conclusion Botulinum toxin appears to confer a specific patient-reported benefit through synkinesis reduction rather than generalised improvement in facial disability. Benefit increases with cumulative treatment exposure and appears to persist up to 12 sessions, providing clinically useful guidance for counselling, treatment sequencing, and long-term FNP rehabilitation after HNC treatment.