You feel terrible, it's been four days, and you'd like something stronger than tea and paracetamol. The instinct is to ask for antibiotics. Often the doctor says no, and it can feel like being fobbed off.
It usually isn't. Antibiotics and antivirals are different kinds of medicine for different kinds of germs, and taking the wrong one does nothing for the illness while still carrying side effects. This guide explains the difference in plain terms and why the instructions on each matter. It doesn't tell you whether you need either; only a clinician who has assessed you can make that call.
Two different kinds of germ
Most of the infections people pick up day to day are caused by either bacteria or viruses.
- Bacteria are single-celled organisms that can live and multiply on their own. Some cause infections, such as strep throat, whooping cough and urinary tract infections, which are the examples the CDC gives.
- Viruses are much smaller and can only multiply inside your own cells. They cause colds, flu, most sore throats, and most chest colds (bronchitis).
The symptoms can overlap a lot. A sore throat, a fever and feeling wiped out can come from either. That's why a clinician sometimes needs to examine you or run a test rather than going on symptoms alone.
What antibiotics do
Antibiotics work against bacteria. They target parts of bacterial cells, killing them or stopping them from multiplying.
The CDC is blunt about the limit: antibiotics do not work on viruses. It specifically lists colds and runny noses, most sore throats (other than strep), flu and most cases of bronchitis as illnesses antibiotics won't help. Even some bacterial infections get better without them.
Taking an antibiotic for a viral illness won't make you better faster. What it can do is cause side effects. The CDC lists common ones such as rash, dizziness, nausea, diarrhoea and yeast infections, and more serious ones including C. diff infection, which causes severe diarrhoea, and severe allergic reactions.
Why "just in case" antibiotics are a problem
There's a second cost that doesn't show up for you personally in the moment: antimicrobial resistance. Every time bacteria are exposed to an antibiotic, the ones that survive can pass on the ability to survive it. Over time, infections become harder to treat. The CDC lists antimicrobial-resistant infections among the serious harms linked to antibiotic use.
This is the main reason clinicians are careful about prescribing antibiotics, and why a "no" for a cold is good medicine rather than a brush-off.
What antivirals do
Antivirals work against viruses, but not in the broad way people sometimes assume. There isn't a general antiviral for "a virus". Each one works against particular viruses. The CDC notes, for example, that flu antiviral drugs only work to treat flu, and that antivirals prescribed for COVID-19 are not approved or recommended for flu.
Flu is the example most people meet. According to the CDC, flu antivirals are prescription medicines, and they work best when started within one to two days of flu symptoms beginning, although starting later can still help some people, especially those who are very ill or at higher risk of complications. The CDC says they can make symptoms milder and shorten the illness by about a day, and can reduce some complications.
A few things follow:
- Timing matters. If you're in a higher-risk group and think you have flu, contacting a clinician early gives them the option.
- They're not a replacement for vaccination. The CDC is explicit that antivirals are not a substitute for a flu vaccine.
- They're not antibiotics. They work differently and treat different things.
So what helps a cold or flu?
For most healthy people with a cold or uncomplicated flu, the answer is time and looking after yourself: rest, fluids, and over-the-counter products for symptoms if you need them. If you use cold and flu products, check the active ingredients so you don't double up on the same one across two boxes. Our guide to common over-the-counter medication mistakes explains how.
Seek medical advice if you're getting worse rather than better, if symptoms last longer than you'd expect, or if you have trouble breathing, a very high temperature, or a condition that puts you at higher risk. If you're not sure, a pharmacist is a good first stop.
Taking a course as prescribed
If a clinician does prescribe an antibiotic or an antiviral, the instructions are part of the treatment. The CDC's advice on antibiotics is to take them exactly as prescribed, and not to share them or save them for later.
In practice that means:
- Follow the schedule on the label. If it says every 8 hours, that's a fixed interval, which isn't the same as three times a day at mealtimes. The dose interval calculator lays out the actual clock times.
- Take it for as long as you were told to. Don't stop early or carry on longer on your own. If you're unsure how long the course is, or you think it isn't working, ask your prescriber or pharmacist.
- Don't keep leftovers. Leftover antibiotics may be the wrong medicine, the wrong amount or out of date next time. Ask your pharmacist how to dispose of them.
- Don't take someone else's. A medicine prescribed for another person's infection may be wrong for yours.
- Ask about missed doses. What to do depends on the medicine and how late you are. The leaflet usually says, and your pharmacist can tell you.
A short, strict course is where reminders earn their keep. Our post on choosing an antibiotic reminder app covers what that job needs: fixed intervals, a real end date, and reminders you can't miss. Doz groups medications by prescription, including how long the course runs, and supports Critical Alerts so a dose reminder can come through even when your iPhone is on Silent.
Frequently Asked Questions
Do antibiotics work on viruses?
No. Antibiotics work against bacteria, not viruses. The CDC states that antibiotics don't work on colds, runny noses, flu, most sore throats (other than strep) or most chest colds. Taking them for a virus won't help and can cause side effects.
What is the difference between an antibiotic and an antiviral?
An antibiotic treats bacterial infections. An antiviral treats infections caused by specific viruses, and each antiviral only works against particular viruses, such as flu. They're different medicines and aren't interchangeable.
Can antivirals cure the flu?
Flu antivirals don't cure flu instantly, but the CDC says they can make symptoms milder and shorten illness by about a day, and they work best when started within one to two days of symptoms. They need a prescription. They're not a substitute for the flu vaccine.
Why won't my doctor give me antibiotics for a cold?
Because colds are caused by viruses, and antibiotics don't work on viruses. Taking them would expose you to side effects without helping you recover, and it contributes to antibiotic resistance. Your doctor may suggest ways to ease symptoms and tell you what to watch for.
Should I finish my antibiotics if I feel better?
Take the course as your prescriber instructed, for as long as they said. Don't change it on your own. If you have questions about how long to take it, or you're getting side effects, ask your prescriber or pharmacist.
Where to start
If you're ill now, work out whether it's the kind of illness that needs a clinician. A pharmacist can help with that. If you've already been prescribed an antibiotic or antiviral, put the schedule into a reminder straight away, with the end date, and keep the leaflet somewhere you'll find it if you miss a dose.
Sources
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