Aim: comparison of bilateral versus posterior injection of botulinum toxin in the internal anal sphincter for treatment of 
acute anal fissure and prevention of chronicity. 
Methods: Forty patients with acute anal fissure were randomly divided into two equal groups. Group I patients were 
treated by injecting 20 units of Botulinum toxin on each lateral side of the internal anal sphincter. Group II patients 
were treated by injecting 25 units of Botulinum toxin in the midline posteriorly. 
Results: Mean period for complete pain relief was 8.45±7.41 days in group I and 7.20±7.19 days in group II. Mean time of healing was 5.20 ±1.85 weeks in group I and 5.40 ±2.01 weeks in group II. Two patients (10%) in group I and 3 patients (15%) in group II showed partial healing. Fissure recurred to 4 patients (20%) in group I and 3 patients (15%) in group II. 
Conclusion: Botulinum toxin injection is effective in treating acute anal fissure and preventing chronicity. A single 
posterior injection is easier, less painful and as effective as bilateral injection in pain relief.