When you start feeling better, some bacteria causing the infection may still be alive even though symptoms have eased. Stopping antibiotics early can leave these survivors behind, allowing the infection to return, sometimes harder to treat than before.
Incomplete courses are also linked to antibiotic resistance, where bacteria adapt and stop responding to the medicine. This is a serious global health concern that affects not just you but the wider community, because resistant infections are more difficult to cure.
Follow the course length your prescriber gave you. Guidance on ideal course lengths is evolving, so if you have side effects or questions, speak to your doctor or pharmacist rather than stopping on your own. They can advise whether it is safe to change the plan.