You treat cacti with pharmaceutical products, but they wither?
The culprit is popular “life hacks” from the Internet that destroy plants.
For example, iodine, which is recommended to combat fungus, burns the roots. Just a drop of a 5% solution makes the soil toxic for a month.

Top 3 dangerous pharmaceutical drugs
1. Activated carbon - clogs drainage holes, creating a "swamp" in the pot.
2. Aspirin - salicylic acid kills symbiotic fungi that help cacti absorb water.
3. Hydrogen peroxide - destroys root hairs, turning the plant into a “disabled person”.
What to replace it with?
To disinfect cuts, use bee propolis. Dilute 1 g of propolis in 100 ml of warm water, apply with a brush to the wounds. It not only disinfects, but also stimulates regeneration.
Instead of activated carbon, add crumbs from ceramic pots to the soil. It does not stick together and provides aeration.
Experiment
Cacti watered with chamomile infusion (1 filter bag per glass of boiling water) grow 20% faster.
Chamomile contains chamazulene, a substance that strengthens cell membranes. But don't overdo it: water no more than once a month.
If you have already poured peroxide on the plant, rinse the roots in distilled water and transplant into dry soil. The first watering is in 10 days. The chance of survival is 50%.