Antibiotics kill harmful bacteria but also affect the helpful bacteria in your gut, which can lead to loose stools or diarrhoea. This is one of the most common side effects and is usually mild, resolving after the course ends.
To cope, drink plenty of fluids to stay hydrated, and eat as normally as you can. Some people find probiotics or live yoghurt helpful, though evidence varies; ask your pharmacist before adding anything.
Contact a doctor urgently if you have severe, watery, or bloody diarrhoea, stomach cramps with fever, or diarrhoea that continues after finishing the antibiotic, as this can occasionally signal a more serious gut infection such as C. difficile that needs specific treatment.