Using Antibiotics

We sometimes prescribe antibiotics to help fight infections. Antibiotics work against bacteria, but not viruses. As a general rule oral contraceptive pills will not work whilst you are on antibiotics and for up to a week after stopping.

How they work

The bacterial cell has a double layer on its outside. The outermost layer – the "cell wall" -- is similar to the outer layer of plant cells, but is missing in human and animal cells. This wall must grow along with the cell, or the growing cell will eventually become too big for the wall and burst and die. Penicillin and cephalosporins kill bacteria by interfering with the wall-building system. Since we don't have cell walls, and plants have a different wall-building system, neither we, nor animals, nor plants are affected by the medicine.

Penicillins and cephalosporins

Penicillins and cephalosporins usually don't cause many problems for a patient. Like all antibiotics, they can cause mild side effects like diarrhoea. Less common side effects include rashes (which may or may not imply a true allergy) and hives (which usually means you're allergic to the medicine). The rarest side effect is "anaphylactic" allergy, in which your airway swells up when you take a dose of the medicine, sometimes to the point where you can't breathe. Although the reaction can be treated if you are close to help, the safest thing if you are that allergic to the medicine is never to take it at all. We tend to prescribe Amoxicillin as our antibiotic of choice from this group.


Erythromycin is another antibacterial produced by a mould. Erythromycin-like antibiotics are also known as macrolides. Erythromycin works by blocking the bacterial cell's machinery for making new proteins. Since proteins are both much of the cell's structure and make the enzymes that direct all the cell's chemical reactions, this makes the cell unable to function. Erythromycin in low doses will stop bacteria from growing and multiplying, but you need a higher concentration to actually kill the bacteria. However, if you can stop growth until your immune system kicks in. Since all protein making is affected, erythromycin can slow down or kill any bacteria, even those without cell walls. The biggest problem with these medicines is that they will irritate the stomach. Always take erythromycin with food or milk!


We use this antibiotic to fight gum infections. It is especially important that we know if you are taking any of the following: Anticoagulants (blood thinners)--Patients taking anticoagulants with metronidazole may have an increased chance of bleeding Disulfiram (e.g., Antabuse)--Patients taking disulfiram with metronidazole may have an increase in side effects affecting the central nervous system Lithium --Patients taking lithium with metronidazole may have an increased chance of experiencing lithium toxicity If this medicine upsets your stomach, it may be taken with meals or a snack. If the stomach upset (nausea, vomiting, stomach pain, or diarrhoea) continues, check with us. Drinking alcoholic beverages while taking this medicine will cause stomach pain, nausea, vomiting, headache, or flushing or redness of the face. Other alcohol-containing preparations (for example, elixirs, cough syrups, tonics) may also cause problems. These problems may last for at least a day after you stop taking metronidazole. Also, this medicine may cause alcoholic beverages to taste different. Therefore, you should not drink alcoholic beverages or take other alcohol-containing preparations while you are taking this medicine and for at least 3 days after stopping it!