The effect of botulinum toxin injection dose on the appearance of surgical scar Early postoperative injection of botulinum toxin type A (BTxA) can reduce surgical scar hypertrophy. BTxA injection at different time points is associated with different levels of efficacy, but the efficacy of different doses of BTxA for scar management has not investigated. The purpose of this study was to investigate the effect of different doses of BTxA administered early after surgery on scar improvement through a split-scar experiment. The study included 22 patients who underwent surgery between September 2019 and October 2020. High- and low-dose BTxA was randomly administered into each half of the surgical wound closure immediately after surgery. One half of the incision was injected with a low dose (4 U) of BTxA, and the other half was injected with a high dose (8 U). The scars were then evaluated at postoperative 6 months using the modified Stony Brook Scar Evaluation Scale (mSBSES), and patient satisfaction was evaluated using the Visual Analogue Scale (VAS). The occurrence of complications or adverse events was also recorded. Twenty patients completed the study and were analyzed. Compared with the low-dose sides, the high-dose sides had significantly better mSBSES scores and significantly higher VAS scores (p < 0.01, respectively). No serious adverse reactions or post-injection complications were observed. Immediately after the operation, high-dose BTxA (that is within the therapeutic range) injection improved the appearance of postoperative scar more than low-dose injection In patients who undergo invasive surgical procedures, the cosmetic scars that appear after wound healing can cause distress1. Multiple factors are associated with undesirable scars, including the surgical technique, anatomical regions, skin tension, postoperative infection, and immunologic responses2. Early treatment of surgical scars can result in better appearance and decrease the need for treatment in later stages3. Various treatments, such as compression therapy, radiation therapy, silicone gel therapy, and laser therapy, have proven to be helpful, but most of these therapies have been unsatisfactory4. In the last ten years, several studies have indicated that botulinum toxin type A (BTxA) has a positive effect on the prevention and treatment of scars, and these include human studies too5–8. For example, Gassner et al. reported that BTxA can fix the underlying muscle tissue to reduce wound tension during scar formation9. Further, Xiao et al. confirmed that continuous injection of BTxA can reduce the thickness and amount of collagen deposition and decrease the degree of hypertrophic scarring10. A proven early treatment method to prevent scar formation is injection of BTxA into surgical lesions at different postoperative times11. In different studies, the injection time of BTxA has been reported from immediately after surgery to 9 days after surgery12–14. Hu et al. showed that BTxA may be more beneficial in the early stages of wound healing, and that injection of BTxA immediately after wound closure can provide excellent results for facial surgical scars15. However, the effects of different doses of BTxA has not yet been studied with a split-scar experiment. Therefore, this prospective, split-scar, randomized controlled trial was performed to investigate the effect of different doses of paralesional BTxA administration on scar cosmesis after surgical excision. Go to effect of botulinum toxin injection