To compare the proportion of cervical dystonia (CD) patients treated exclusively with complex-free (incobotulinumtoxinA; CF) or complex-containing (onabotulinumtoxinA or abobotulinumtoxinA; CC) botulinum toxin type A who experienced a worsening in dose–effect (DEff) at year 7 versus year 2.

Repeated botulinum toxin type A (BoNT/A) injections may reduce clinical benefit over time. RELY‑CD introduced DEff, a parameter integrating dose adjustments and treatment effect to identify treatment attenuation (DEff > 1.2), accounting for real-world variability in efficacy scales.

RELY‑CD is a multicenter chart review from 13 sites in Germany, Spain, and Poland. Adults (18–64) with CD and at least 7 years of BoNT/A treatment were included. The monotherapy analysis set comprised patients treated exclusively with one BoNT/A category (CF or CC). The mean of year 2 visits served as reference; year 7 was the primary endpoint. DEff was calculated as the product of a dose change coefficient (per-muscle dose change) and an effect change coefficient (year-specific effect vs reference). A 20% worsening (DEff > 1.2) was chosen as cut-off (considered clinically meaningful in similar settings). Descriptive analyses were pre-specified; data from years 5 and 10 were collected optionally

Among 250 eligible patients, 128 were monotherapy (CF n=27; CC n=101). At year 7, DEff > 1.2 occurred in 11.1% (3/27) CF vs 24.2% (24/99) CC. Clinician-reported signs of treatment attenuation were less frequent with CF than CC: dose increase 3.7% vs 19.8%; effect duration decrease 3.7% vs 16.9%; decrease in efficacy 3.7% vs 16.9%; patient-reported dissatisfaction 3.7% vs 6.0%; lack of efficacy 0.0% vs 3.0%.

A higher proportion of CC-treated patients showed worsening in dose–effect by year 7. While a 20% threshold is commonly used in clinical research, its relevance for DEff requires further validation. These findings support the utility of DEff for longitudinal assessment and suggest formulation-related differences that merit prospective confirmation.