Amoxicillin is a prescription antibiotic similar to penicillin, and is used to treat bacterial infections in various parts of the body. It can be taken as a pill, powder, liquid or injection depending on the patient, and is available under a variety of brand names¹ in many countries.

Where Does Amoxicillin Come From?

Amoxicillin is related to penicillin, which was first discovered by Sir Alexander Fleming in 1928 when he noticed it being produced by the mould Penicillium notatum. However, amoxicillin is an artificially altered (semisynthetic) variant of penicillin, and was first made in 1972. The whole point of converting penicillin into amoxicillin is that the latter is more effective at treating disease. Augmentin (See 'Clavulanic Acid' below) was first introduced in 1998, and is even more useful.

What Does It Do and Why?

Amoxicillin doesn't actually kill bacteria, but instead it prevents them from multiplying, thus making it easier for the immune system to wipe out the infection. It does this by stopping the new bacteria from forming their cell walls, which are necessary to avoid the contents of the bacterial cell from spewing out into its surroundings. The result of all this is that the infection gradually dies out from the lack of a new generation to replace the old one.

The mechanism by which it prevents the cell walls from forming is based on what is known as the receptor concept. This is where a drug binds to a receptor on the outside of the cell, activating it, thereby causing changes in the cell. In the case of amoxicillin, the receptor is a penicillin binding protein (PBP) on the cell wall of the bacteria. When amoxicillin attaches itself to the PBP, it causes a halt in the production of one of the ingredients of the cell wall, peptidoglycan. Peptidoglycan provides the cell wall with stability, and a lack of it weakens the cell wall to the point of collapse. Altogether, this causes the bacterial cell to split and fall apart.

The Side Effects of Amoxicillin

Contrary to popular belief, all drugs do in fact have a range of other possible effects, as they don't just affect the targeted receptor. This means that although amoxicillin will have the desired effect of halting the bacterial infection, it may also interact in a minor way with the patient's own body. The antibiotic also kills off many of the good bacteria lining your gut, which can lead to a nice bout of diarrhoea. Some patients can become allergic to the drug due to an immune reaction².

Side effects of amoxicillin can include abdominal pain, allergic reactions, bleeding, confusion, diarrhoea, dizziness, easy bruising, heartburn, itching, insomnia, nausea, paleness or yellowing of the skin, rashes, tiredness or vomiting.

These may seem quite nasty, but the drug has beneficial effects which are far greater than the risks and unpleasantness attached to the side effects. Indeed, it is quite possible to take a course of amoxicillin without experiencing any drastic side effects at all. An overdose of the drug, however, will almost certainly be detrimental³.

One much more important effect caused by many antibiotics is a reduction in the effectiveness of oral contraceptives. It is therefore advisable to use additional contraceptive protection during and after a course of amoxicillin.

When and How is Amoxicillin Used?

Amoxicillin is used to treat bacterial infection of the ear, nose, throat, skin and urinary tract, as well as pneumonia, bronchitis and gonorrhoea, which are caused by bacteria which are susceptible to amoxicillin. The bacteria it is used against are all linked by their inability to destroy the amoxicillin before it destroys them. The drug will not work against resistant strains of bacteria such as MRSA, which have evolved from susceptible bacteria. These substrains are resistant because they produce penicillinase⁴, an enzyme which breaks up antibiotics of the penicillin family, including amoxicillin. As mentioned above the drug destroys bacterial cell walls, and so will obviously have no effect whatsoever on viruses.

Amoxicillin is most often taken with a light meal as a pill, or a powder mixed into water by the pharmacist, but can also be given as an injection by a doctor. The drug must first reach the site of the infection, and this takes about half an hour when taken orally. The drug is eventually filtered out of the body by the kidneys and expelled in the urine, with the level of the drug in the blood halving every hour. For this reason you need to take the drug regularly as directed by your doctor. Also, it is very important to finish the entire course of antibiotics so as to kill all the bacteria and thereby reduce the chance of an antibiotic-resistant strain evolving.

Clavulanic Acid

Clavulanic acid is produced by the yeast strain Streptomyces clavuligerus, and is added to amoxicillin in drugs such as Augmentin. It inactivates the enzymes which amoxicillin-resistant bacteria produce to break down the drug, enabling amoxicillin to get on with its job of destroying the infection without itself being destroyed by the bacterium's defence mechanism.

Should I Be Taking Amoxicillin?

This is something for a trained professional to decide, as not everyone can take the drug, and the risks will outweigh the benefits if the drug is taken needlessly. There is also the growing problem caused by the increasing resistance of bacteria to antibiotics, leading doctors to only give drugs such as amoxicillin when they are genuinely necessary. Patients should therefore visit their doctor because they are ill, not because they have read about an antibiotic and feel like trying it.

Please Note: h2g2 is not a definitive medical resource. If you have any health concerns you must always seek advice from your local GP. You can also visit NHS Direct or BBC Health Conditions.