PURPOSE: Cosmetic botulinum toxin A (BoNT-A) injections are increasing worldwide. Adverse effects need to be evaluated in more detail for BoNT-A regulations. This study aims to assess the anticholinergic effect of BoNT-A on accommodation amplitude (AA) after upper face application. METHODS: Twenty patients aged between 20 and 45 years were recruited in this prospective, interventional study. Abobotulinum toxin A of 500 units was diluted with 3 ml saline and injected into the forehead, glabellar and periorbital area. AA was measured with both the push-up and minus-lens techniques before and after two weeks of the injections. T-test for normally distributed variables was applied for the comparison of the results. RESULTS: The study group comprised 18 women and 2 men with a mean age of 33.9 ± 8.59. AA decreased significantly after two weeks of the injection. In the push-up test, AA was 9.18 ± 4.72 D before BoNT-A injection and decreased to 7.11 ± 3.02 D after the injection (p <
0.001). In the minus lens test, AA reduced from 5.86 ± 2.24 D to 5.24 ± 2.06 D after BoNT-A injection (p <
0.001). Refraction did not change in any of the participants. CONCLUSION: The present study showed that cosmetic upper face BoNT-A injections reduced AA. However, this reduction did not result in any significant symptomatic effects in the patients. These findings suggest that while BoNT-A injection has a measurable impact on the ciliary muscle or its parasympathetic innervation, it may not lead to noticeable clinical outcomes.