Dry, hard stools and a fissure after finishing a course of antibiotics are common, since antibiotics can disturb your normal gut bacteria for weeks afterward. Soluble fibre alone often isn't enough once that balance is disturbed.
Next Steps
A multi-strain probiotic with Lactobacillus and Bifidobacterium is generally the type that helps restore gut flora, alongside plenty of water and a mix of soluble and insoluble fibre. Keep the fissure area clean and use a stool softener if the bathroom visits still hurt.
Health Tips
See a physician or gastroenterologist if the fissure doesn't heal in two to three weeks, or if you notice bleeding or worsening pain.