Developed by


By Bjarne Lühr Hansen PhD, MD and Philipp Skafte-Holm MD, Mentor Institute

Antibiotics are used for infections caused by bacteria but has no activity against viral infections. The doctor can examine you and thereby determine whether you need antibiotics. If you are still sick and have a fever after 3-4 days of antibiotics you should consult a doctor again. You should always take antibiotics as long as your doctor has prescribed. The most common adverse reactions to antibiotics are thin stools, stomach cramps, nausea and anorexia.

Many infections cause fever but far from everyone with a fever needs antibiotics.

Antibiotics are a remedy that kills bacteria and are therefore only used for bacterial infections. Since far most infections are caused by virus, only few of those with infections need antibiotics. Common infections like common cold and influenza are caused by virus and therefore cannot be treated with antibiotics.

Illnesses like tonsillitis, pneumonia and sinusitis can be caused by bacteria but in most cases it is a virus that is at work and the antibiotics are therefore ineffective. It can be difficult to determine whether an inflammation is caused by bacteria or virus. The doctor can examine you and determine whether you need antibiotics.

The most well-known antibiotics is probably penicillin but there are many different kinds of antibiotics. There are broad-spectrum (kills many different bacteria) and narrow-spectrum (kills a few selected bacteria) antibiotics. As a rule of thumb, the doctor starts with narrow-spectrum antibiotics – for example penicillin. The most bacterial infections can be cured with penicillin. Some bacteria change during treatment and become resistant towards the employed antibiotic. In those cases, the doctor has to change to a different antibiotic.

After 2 to 3 days on antibiotics, the fever disappears and you feel better. If you are still ill after 3 to 4 days on antibiotics, the medication is ineffective. The explanation may be that the chosen antibiotic does not kill the bacteria and in that case, the doctor may choose a different antibiotic. Another – very frequent – explanation is that it is not a bacterial infection but a viral infection – and in that case, antibiotics are ineffective.

Most infections should be treated for at least 6 days, even though you recover after a couple of days. If you stop the treatment too soon, the infection will return with full power. You must always take the antibiotics for as long as the doctor has ordered.

The most common side effects from antibiotics are loose stool, stomach ache, nausea and loathing of food. Very few people are allergic to penicillin and react with itching rash and difficulty with breathing, if they have penicillin. The doctor will then choose a different antibiotic. You should always tell your doctor, if you have reacted with rash and difficulty in breathing on penicillin before.

Further reading on Fever.



Most bacterial infections transmit through spit and secrete. After 2 days treatment with antibiotics, the risk of contagion is minimal.

When can I go to work?
It depends on which infection you have.

What can you do?

When you have a fever you should dress lightly, stay in a cool room and drink plenty.

Contact the doctor tomorrow

If the fever has not diminished, after 2 days treatment with antibiotics.

Contact the doctor immediately

If you are under treatment with antibiotics and are feeling worse.