Antibiotics upset my stomach. What can I do about it?

An upset stomach is a common side effect of antibiotics. Loose stools (diarrhoea), feeling sick, bloating and indigestion can all happen, and the UK's NHS says these side effects are usually mild and should pass once you finish your course. In the meantime, drink plenty of fluids, eat when you feel able to, keep taking the medicine as directed, and ask the doctor who prescribed it or a pharmacist if the side effects bother you.

Why can antibiotics upset your stomach?

Antibiotics are medicines that act on bacteria. Like any medicine they can have side effects, and the stomach and bowel are where many people notice them.

The common side effects

The NHS lists the common side effects of antibiotics as being sick, feeling sick, bloating and indigestion, and diarrhoea. It also says that most antibiotics do not cause problems if they are used properly and that serious side effects are rare. The US CDC also lists nausea and diarrhoea among the common side effects of antibiotics.

The Faculty of Pharmacy at Mahidol University in Bangkok notes that every antibiotic has some side effects, and that how much you notice depends on the type of medicine, the dose, how it is taken and how sensitive each person is to it. Diarrhoea is one of the side effects it lists as common. This is why two people taking the same medicine can have quite different experiences.

Bacteria also live in the gut

MedlinePlus, the health information service of the US National Library of Medicine, explains that antibiotics work by killing bacteria or making it hard for them to grow and multiply, and the NIDDK, part of the US National Institutes of Health, explains that bacteria in the large intestine also take part in breaking down food. This may be one reason the gut can feel different while you are taking them.

How long does an upset stomach from antibiotics usually last?

The NHS says it usually passes once you finish the course. What health agencies say is summarised below.

Everyone is different

The NIDDK lists medicine side effects among the most common causes of short-term diarrhoea, and names antibiotics as one example. It advises people who have diarrhoea while taking antibiotics to keep in touch with their doctor. How long it lasts varies from person to person. If it carries on, or if you are unsure, check with a pharmacist or doctor rather than waiting.

What can I do while I'm taking antibiotics?

The basics are fluids, food you can manage and rest. None of this replaces the advice of the doctor or pharmacist who gave you the medicine.

Drink enough fluids

Loose stools mean you lose more water than usual. The NHS advice for looking after yourself with diarrhoea and vomiting is to drink lots of fluids, such as water, and to take small sips if you feel sick. It advises not having fruit juice or fizzy drinks, as they can make diarrhoea worse.

The NHS page on dehydration adds that a pharmacist can recommend oral rehydration solutions (ORS), which are powders you mix with water, and suggests avoiding caffeinated drinks and alcohol because both will dehydrate you.

Eat when you feel able to

The NHS suggests eating when you feel able to, and says it may help to avoid foods that are fatty or spicy. The NIDDK says that you may not feel like eating, and that in most cases you can eat your normal diet when you feel like eating again. It adds that most experts do not recommend a restricted diet or fasting beyond avoiding the foods and drinks below.

The NIDDK's eating advice lists these as foods and drinks that could make diarrhoea worse. In Thailand, plain rice soup or rice porridge without much oil is an everyday example of a light meal.

If your stomach feels off after several days of fatty or fried food eaten out, see Eating out for days and my stomach feels off. What can I do?

Rest

The NHS advises staying at home and getting plenty of rest if you have diarrhoea and vomiting. Keep your days light until you feel more like yourself.

Should I stop taking antibiotics if they upset my stomach?

No, not on your own. Keep taking them as prescribed, and talk to the doctor or pharmacist about the side effects.

Take them as directed and ask for advice

The NHS advises taking antibiotics as directed on the packet or the patient information leaflet, or as instructed by your doctor or pharmacist, and contacting your doctor for advice if you get side effects. Only take antibiotics prescribed for you, and never borrow them from friends or family, as the NHS advises.

Check before adding anything else

If you are thinking of taking any supplement or other medicine while you are on antibiotics, ask your doctor or pharmacist first.

When should I see a doctor?

Most upset stomachs from antibiotics are mild. Get help if you notice any of the following.

Signs of dehydration

The NHS lists these signs of dehydration in adults: feeling thirsty, a headache or feeling light-headed, dark yellow, strong-smelling pee, peeing less often than usual, feeling dizzy, feeling tired, a dry mouth, lips and tongue, and sunken eyes. It advises getting urgent advice if you feel unusually tired, feel dizzy when you stand up and it does not go away, have dark yellow pee or are peeing less than normal, or are breathing quickly or have a fast heart rate. The NHS also advises older people, and people with other health conditions that make losing fluids more likely, to speak to a pharmacist.