Diarrhoea can be caused by a poor diet or a stomach bug and should pass in a few days. However there are some medicine to stop diarrhoea for a few hours (like loperamide) and supplement and probiotic to restore your gut environment before, during and after.
First of all it's very important to replenish the fluids lost by drinking rehydration sachets ( Dioralyte) and plenty of water throughout the day.
Please note that these over the counter medications are only intended for the occasional treatment of diarrhoea symptoms.
If unsure of which product to choose ask for our pharmacist's advice.