Healthy Living

Penicillin and Other Drugs in the Same Class

Penicillins Classification, Side effects, Uses

About Penicillin

Penicillin is an antibiotic derived from the Penicillium notatum mold. In 1928, a Scottish bacteriologist named Alexander Fleming discovered penicillin. Since Fleming's discovery, several penicillins have been derived through modifying the mold's original structure to produce enhanced penicillins to fight against different types of bacteria. The other penicillin antibiotics are:

  • Phenoxymethylpenicillin (penicillin V)
  • Ampicillin
  • Amoxicillin
  • Flucloxacillin
  • Cloxacillin
  • Piperacillin
  • Ticarcillin
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Penicillins are antibacterial agents. However, they can also cause side effects, such as skin rash, itching, nausea, or vomiting. Anaphylaxis due to a penicillin allergy is rare. Around 1-5 people in every 10,000 cases develop anaphylaxis due to penicillin therapy. People who are allergic to penicillin and those who develop anaphylaxis should not be given other types of penicillin as well. In some cases, people who develop anaphylaxis due to penicillin may also be allergic to certain types of cephalosporins.  


A number of penicillin derivatives have been developed these days. They inhibit the growth and multiplication of a wide range of bacteria than the first created antibiotic. Although most Staphylococcus species are penicillin-resistant, penicillin itself is still effective in eliminating the following types of bacteria: 

  • Clostridium
  • Listeria
  • Streptococci (Streptococcus pneumoniae)
  • Neisseria gonorrhoeae
  • Peptostreptococcus

Other types of penicillin antibiotics are also used to treat infections caused by the following bacteria:

  • Certain strains of Staphylococci
  • Pneumococcus
  • Escherichia coli or E. coli
  • Haemophilus influenzae
  • Salmonella
  • Shigella
  • Pseudomonas aeruginosa

Penicillins are widely used for the treatment of different types of bacterial infections, such as those that affect the sinuses, middle ear, bowels, kidney, and bladder. These antibiotics can also be used in treating:

How does it work?

Penicillin antibiotics stop bacteria from growing or multiplying. Penicillin works by preventing bacteria from forming their cell walls, which surround them. Their cell wall serves as a protective barrier, which maintains the cell's shape and prevents lysis from osmotic pressure. Without the presence of a cell wall, bacteria would not be able to survive. Penicillin antibiotics are said to be the most effective treatment when bacteria are trying to multiply and form their cell walls.

Penicillin Antibiotics

Below is a list of other penicillin antibiotics that are also available in the United States:

  • Amoxicillin (Amoxil)
  • Ampicillin (Unasyn)
  • Cloxacillin
  • Dicloxacillin (Dycill)
  • Penicillin V
  • Penicillin G (Pfizerpen or Permapen)
  • Piperacillin (Pipracil)

Classification of Penicillin

1. Aminopenicillins

Aminopenicillins are bactericidal beta-lactam antibiotics, which inhibit the synthesis of the bacterial cell wall. This type of penicillin is quite similar to the penicillin drug, but has a broader spectrum of antibiotic activity than the original penicillin drug. Aminopenicillins can be orally administered since they are not inactivated by acid hydrolysis.

They are said to be quite effective for the treatment of infections caused by gram-positive bacteria and gram-negative bacterial infections, such as Haemophilus influenzae and Escherichia coli. Aminopenicillins are mostly used for the treatment of upper as well as lower respiratory tract infections, urinary tract infections (UTIs), and skin infections, among others. 

Examples of aminopenicillin are amoxicillin (Moxilin, Amoxil, Amoxicot, Moxatag, Apo-Amoxi, DisperMox, Trimox) and ampicillin (Principen, Totacillin-N, Omnipen-N). 

2. Antipseudomonal Penicillins

These are basically antimicrobial agents that are mostly used for the treatment of pseudomonal infections. They are said to be very similar in structure to aminopenicillins, but have either the carboxyl group or the urea group as instead of amines. Some examples of antipseudomonal penicillins are carbenicillin, ticarcillin, and piperacillin. 

3. Natural Penicillin

The first antibiotics used in clinical practice were natural penicillins. These antibiotics were based on the structure of the original penicillin G. Generally, natural penicillins are bactericidal and particularly inhibit the synthesis of bacterial cell walls. 

Natural penicillins are effective for the treatment of infections caused by gram-positive bacteria, such as streptococci and staphylococci, including gram-negative bacteria such as TreponemaLeptospiraBorrelia, and meningococcus. 

Below is a list of natural penicillins:

  • Penicillin G benzathine (Bicillin L-A, Isoject Permapen)
  • Penicillin V potassium (Penicillin VK, Beepen-VK, PC Pen VK, Pfizerpen, Veetids)
  • Procaine penicillin (Wycillin)

4. Penicillinase-Resistant Penicillin

These are said to be antibiotics that are mostly inactivated by the penicillinase enzyme. Some bacteria are said to produce the penicillinase enzyme, which has the tendency to destroy the beta-lactam ring of the antibiotic, thereby making penicillins ineffective. They are mostly used for the treatment of resistant strains of staphylococci as well as other kinds of infections.

Methicillin was considered as the first member of this group. It was later followed by nafcillin, dicloxacillin, oxacillin, and cloxacillin. Methicillin was the first penicillin drug that was developed through a specific type of modification called rational drug design. Since then, penicillin-resistant bacteria have been designated as "methicillin-resistant".

Below is a list of penicillinase-resistant penicillins:

  • Oxacillin (Bactocill)
  • Dicloxacillin (Dycill, Dynapen)
  • Nafcillin (Unipen)

Side Effects 

The following are some of the side effects of penicillin:

There are also reported cases of people who develop allergic reactions due to cephalosporins, which are related to penicillins. Examples of cephalosporins are cephalexin (Keflex), cefaclor (Ceclor), and cefprozil (Cefzil).

Penicillins may also cause serious side effects, which include:

Apart from the ones mentioned above, some may also suffer from convulsions or neurologic troubles when larger doses of parenteral penicillin are taken. Such cases can be seen in patients who already have a renal dysfunction. The intake of ticarcillin can also lead to the occurrence of hypokalemic metabolic alkalosis. With the intake of nafcillin or methicillin, individuals may suffer from interstitial nephritis.

Penicillin Formulations

Penicillin antibiotics are available as capsules, tablets, powder for injection, and powder for oral suspension. 

Drug Interaction

  • Probenecid (Benemid)
  • The combination of ampicillin and allopurinol can increase the risk of developing drug-related skin rashes.
  • The effects of typhoid live vaccine and BCG live vaccine may be reduced by penicillins.

Pregnancy and Breastfeeding

  • Penicillins are safe to use during pregnancy.
  • Small but safe amounts of penicillin may pass into breast milk, making it safe to use while breastfeeding.