Antibiotics are divided into groups depending on how they work in your body and how they kill germs. Within each group there are several different drugs, and some drugs are newer than others. Sometimes your doctor may give you a combination of different antibiotics. 
Here's a brief comparison of some of the antibiotics used to treat pneumonia.Penicillins
These were the first antibiotics. They are still used to treat lots of infections. Occasionally, penicillins can cause an allergic reaction. This happens when the body's natural defences overreact to the drug. People who are allergic to one kind of penicillin may be allergic to others. Penicillins can also cause kidney problems and anaemia in some people, and reduce the number of white blood cells in the body (a condition called leukopenia). White blood cells help fight infections.
Some bacteria that cause pneumonia are becoming resistant to penicillins. Drugs that used to kill certain types of bacteria no longer work, and the bacteria can continue to multiply. Resistance to antibiotics is a growing problem. It is an especially big problem among pneumococci, the bacteria that cause most cases of bacterial pneumonia.     To learn more, see Resistance to antibiotics.
The most common penicillins used to treat pneumonia (with common brand names) are:
The Committee on Safety of Medicines (the group that advises the government on which drugs work and are safe) warns there is a risk of a type of jaundice if you take amoxicillin-clavulanate. If you have jaundice, you get yellow colouring on your skin or in the whites of your eyes. This suggests your liver isn't working as well as it should.Macrolides
These antibiotics are often prescribed for people with pneumonia.
Macrolides can make you feel sick and give you stomach cramps. They can also damage your liver, but this doesn't happen very often. If you take them for a long time, you should have regular tests to make sure your liver is working normally. Macrolides can also interfere with the way other medicines work. Your doctor will check what other drugs you are taking before prescribing macrolides for you.
The most common macrolides used to treat pneumonia (followed by their brand names) are:
The antibiotic telithromycin (brand name Ketek) is similar to erythromycin but acts against a wider range of bacteria. It belongs to a group of antibiotics called ketolides, which are like macrolides. Ketolides were developed because many bacteria were becoming resistant to macrolides. Telithromycin is the only ketolide available so far.
A very small number of people have had liver problems after taking telithromycin.  If you're taking telithromycin and you get any of the following symptoms, you should stop treatment and see your doctor straight away: Cephalosporins