How antibiotics work: Types, uses and side effects

3rd May, 2021 • 13 min read

What are antibiotics?

Antibiotics are medicines that can treat infections caused by bacteria or some parasites.

They work by killing bacteria or stopping bacteria from spreading, giving your immune system more time to clear an infection. But they don’t work if you have an infection caused by a virus, like a cold or flu.

Read on to discover which infections antibiotics treat, when you should and shouldn’t use them, and how to make sure you use them safely, so that they work like they should.

What do antibiotics treat?

Antibiotics are usually only used for more serious bacterial infections, as many mild bacterial infections clear up by themselves.

Antibiotics can be used for bacterial infections that:

  • aren’t likely to go away on their own
  • could be passed on to other people
  • could take too long to go away without treatment
  • could lead to more serious complications

This could include infections like

urinary tract infections
(UTI) and the skin conditions,
. They also treat more serious illnesses like

Antibiotics are used to treat some sexually transmitted infections (STIs) too, like

, which are caused by bacteria.

Sometimes antibiotics are given to prevent, rather than treat an infection – this is known as antibiotic prophylaxis. For example, you may need to take a course of antibiotics before a surgery with a high risk of infection, or if a doctor is concerned a wound may become infected.

But antibiotics can’t kill viruses and won’t make you feel better if you have a viral infection, such as:

  • a
  • flu
  • sinusitis
  • most
    – usually coughs and bronchitis are caused by a virus, but sometimes they can be caused by bacteria and may need antibiotics
  • most
    sore throats
    – both are usually caused by a virus, but sometimes they can be caused by bacteria and may need antibiotics

Antibiotics can’t treat COVID-19
, which is caused by a virus.

What types of antibiotics are there?

There are hundreds of different types of antibiotics – they work in different ways and fight different kinds of bacteria. You may have heard of penicillin, the first antibiotic ever discovered and the most widely-used antibiotic.

Most antibiotics are divided into classes, or families. The main ones include:

  • penicillins, such as penicillin and amoxicillin
  • cephalosporins, such as cephalexin
  • tetracyclines, such as tetracycline and doxycycline
  • macrolides, such as erythromycin and clarithromycin
  • fluoroquinolones, such as levofloxacin and ciprofloxacin
  • aminoglycosides, such as gentamicin

Antibiotics are known as either broad-spectrum or narrow-spectrum antibiotics. Broad-spectrum antibiotics treat infections caused by a wide range of bacteria, while narrow-spectrum antibiotics only fight specific bacteria.

How should you use antibiotics?

Antibiotics can be used in different forms:

  • as creams, ointments, lotions, drops and sprays that you put on your skin (topical antibiotics) – often used to treat skin, eye or ear infections
  • as tablets, pills, capsules and liquids that you swallow (oral antibiotics) – usually used for most mild to moderate infections
  • as an injection or given by vein (intravenously [IV]) – usually used when it’s a more serious infection that needs to be treated in hospital

There are many countries, including the UK, where you can usually only get antibiotics by seeing a doctor first. But, while you have to see a doctor first to get oral antibiotics in the US, there are some topical antibiotics that you can buy from US pharmacies. In some countries, including India, you can buy antibiotics in a pharmacy without seeing a doctor first.

Each antibiotic is effective only against certain bacteria. When choosing the best antibiotic for your infection, a doctor will consider:

  • the kind of infection and how serious it is
  • how strong your immune system is (how well it can help the antibiotic fight the infection)
  • the antibiotic’s possible side effects
  • whether you may have an allergic reaction to the antibiotic
  • any other medicine you’re taking (antibiotics may stop them working as well as they should or cause bad reactions)
  • the cost of the antibiotic
  • if you’ll be able to complete the entire course of antibiotic treatment as you may need to take it often or only at specific times, such as before, during or after meals
  • if you’re
    or breastfeeding

To make sure you take antibiotics safely and so that they work as they should, follow the advice below.

Follow all the advice your health professional gives you about antibiotics

To make sure that antibiotics do their job and kill the infection, it’s very important to follow all the advice a health professional has given you. For example, if you’re taking an antibiotic pill, you may need to take it with food or on an empty stomach.

A doctor, dentist or pharmacist will also tell you how many times a day you need to use an antibiotic and for how long – usually you’ll need to take pills between 1 to 4 times a day.

How long will you need to use antibiotics for?

The length of your treatment will depend on what kind of infection you have, how serious it is and how quickly you get better after starting antibiotics.

Antibiotics are usually given for at least 5 days, although if you have a mild urinary tract infection, you may only have to take them for a few days.

If you have acne, you may be given a topical antibiotic to put on your face once or twice a day, but if this treatment doesn’t work, you may have to take antibiotic pills (oral antibiotics) for a few months.

Some serious infections, like

, may need antibiotics that are given by vein (intravenously [IV]) in a hospital for up to 4 to 6 weeks.

It’s important to finish the entire course of antibiotics you’re given. You may feel better before you’ve finished all the medicine, but you have to complete the course. This makes sure that the bacteria or parasite is completely killed and doesn’t cause another infection – and that the bacteria doesn’t become resistant to that specific antibiotic in the future.

Can you drink alcohol while taking antibiotics?

It makes sense to avoid alcohol when you’re taking medicine and feeling unwell, because drinking alcohol may make you feel worse or increase the side effects of the antibiotics.

But, if you would like to drink, make sure it’s in moderation – alcohol usually won’t cause problems with most common antibiotics. However, if you’re taking metronidazole or tinidazole, don’t drink alcohol until a few days after you finish them – you may get unpleasant side effects, such as:

  • feeling and being sick (
  • hot flushes
  • headaches
  • tummy pain
  • a fast or irregular heartbeat
  • dizziness
  • drowsiness

There are some antibiotics, like doxycycline, that don’t work as well as they should if you drink alcohol with them.

Make sure you read the information leaflet that comes with your medication to find out if you should avoid alcohol or not.

Do antibiotics stop the contraceptive pill from working?

Most antibiotics don’t stop the

combined oral contraceptive pill
(usually called ‘the pill’) from working, so you don’t need to use extra contraception like a condom while taking them.

But some antibiotics, like rifampicin or rifabutin, can affect how well the birth control pill works, so you may need to use an extra type of contraceptive, like condoms, for at least 4 weeks after finishing the antibiotics.

If the antibiotics you’re taking cause side effects like vomiting and diarrhoea, you may not absorb the contraceptive pill as normal, so you should also

use a different type of contraception

Speak to a doctor to find out if you need to use extra contraception while taking antibiotics.

What should you do if you’ve missed a dose or taken a double dose of antibiotics?

If you’ve forgotten to take a dose of antibiotics, you should take that dose as soon as you remember, then carry on taking the rest as normal. But, if it’s nearly time for your next dose, you should skip the missed dose and just carry on taking the rest as normal.

Don’t double up on your next dose of antibiotics as this increases your risk of side effects. If you accidentally take a double dose, it’s unlikely to cause serious problems, but it can increase your risk of side effects and cause mild symptoms like a sore stomach, feeling or being sick (nausea and vomiting), and


If you’ve taken more than 1 extra dose of antibiotics by mistake, are worried or you’re getting serious side effects, speak to a doctor or call emergency services immediately

And if you’ve missed several doses of antibiotics or more than 1 day’s worth of treatment, ask a doctor for advice.

What is antibiotic resistance?

You shouldn’t take antibiotics unless you really need them, as using them when you don’t need them may lead to them not working in the future. This is called antibiotic resistance, and it’s a serious problem worldwide.

Examples of antibiotic-resistant bacteria include:

To prevent the spread of antibiotic resistance, you should:

  • only use antibiotics when a health professional has told you to
  • not demand them if a doctor says you don’t need them
  • always follow a doctor’s advice when taking them, including making sure you complete the whole course of antibiotics
  • never share or use leftover antibiotics
  • prevent infections by regularly washing your hands, making food hygienically, avoiding close contact with sick people, practising safe sex and keeping your vaccinations up to date

What are the side effects of antibiotics?

Some antibiotics may not be safe for you. Tell a doctor or dentist if:

  • you’re allergic to penicillin or any other antibiotic
  • you have other medical conditions, like liver or kidney problems
  • you’re taking other medicines – antibiotics can sometimes interact with other medication, which means it can have an effect that is different to what it’s meant to have
  • you’re pregnant or breastfeeding

Like any medication, antibiotics may cause side effects, but these are usually mild and should stop once you finish your treatment. The most common side effects may include:

If you have side effects, speak to a doctor for advice before you stop taking the medication – they may be able to give you another antibiotic.

Tetracycline antibiotics, including doxycycline, can make your skin sensitive to sunlight and artificial light from, for example, sun lamps or sun beds. Avoid sunlight and artificial light as much as possible while taking it.

You can find out more about the side effects of any antibiotics you’re taking by reading the information leaflet that comes with the antibiotics.

When to see a doctor if you’re taking antibiotics

See a doctor as soon as possible if:

  • your symptoms don’t get better, get worse or come back after treatment
  • you have an itchy rash, skin that gets red and feels hot (flushes), a blocked or runny nose and you feel sick (nausea) – these could be signs of an allergic reaction to antibiotics, especially penicillin and cephalosporins
  • if you’re getting side effects from the medicine
  • if you’ve missed several doses of antibiotics or more than 1 day’s worth of treatment
  • if you develop symptoms you didn’t expect, you feel generally unwell or are worried about your symptoms

You should call an ambulance or go to an emergency department if:

  • your face, tongue, eyes or anywhere else start swelling; you have trouble breathing or cough or wheeze; you feel dizzy or light-headed, collapse or pass out, have a fast heartbeat, clammy skin, or feel confused and anxious – these could be symptoms of a serious allergic reaction called
  • you have watery diarrhoea, blood or pus in your diarrhoea, fever, bad tummy pain – these could be signs of the serious gut infection, clostridium difficile
  • you’ve taken more than 1 extra dose of antibiotics by mistake and you’re getting serious side effects

Your health questions answered

Can I take 2 different antibiotics together?

Doctors may sometimes give you 2 antibiotics to take together. Different antibiotics kill different bacteria. So, by combining antibiotics, doctors can treat a wider range of bacteria that are causing an infection, compared to only using 1 antibiotic that may not work as well. Some bacteria can also be more resistant to 1 antibiotic but not another. By combining 2 different antibiotics, doctors can also increase the chances of successfully treating antibiotic-resistant bacteria. – Answered by Dr

Aleem Qureshi
from the
Healthily Medical Team

Can you get rid of a bacterial infection without antibiotics?

Yes, not all bacterial infections need antibiotics and many mild bacterial infections get better on their own. In fact, you shouldn’t take antibiotics unless you really need them. If a doctor advises you to take antibiotics, it may be because your bacterial infection is unlikely to go away or could take too long to go away without antibiotics. Other reasons could be that you could infect others or your infection may lead to more serious complications if you don’t take antibiotics.

What do antibiotics do to your body?

Antibiotics fight infections caused by certain kinds of bacteria and parasites. They do this in 1 of 2 ways: they either kill the bacteria or stop it from multiplying and growing in your body. When you take antibiotics, it affects the ‘good’, or healthy bacteria, as well as the ‘bad’ bacteria in your gut, which can cause diarrhoea. Like all medicines, antibiotics can have side effects, but usually they’re mild and go away once you stop the medication. But there are more serious side effects that will need immediate medical attention, including a severe allergic reaction called anaphylaxis. Speak to a doctor if you’re worried about any side effects.

Key takeaways

  • antibiotics are medicines that fight bacterial infections; they don’t work for viral infections
  • you shouldn’t take antibiotics unless you need them – this can lead to antibiotic resistance
  • certain antibiotics may not be safe for you if you have a medical condition, are pregnant or breastfeeding, or take other medication
  • antibiotics can cause side effects, but these are usually mild and should stop once you’ve completed your treatment
  • some people have an allergic reaction to antibiotics – call the emergency services immediately if you think you, or someone else, is having a serious allergic reaction (anaphylaxis)

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.