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Clinician’s Corner: Penicillins

Penicillins have a most interesting history. We are familiar with the story of Sir Alexander Fleming discovering that moldy bread had produced a substance that killed off bacteria, which became known as penicillin. I am amazed to read the stories of how the United States “ramped up” the manufacturing of penicillin to meet the demands of World War Two.  It was indeed an international effort between the United States and the Great Britain.  Great Britain had the scientists and the technology, but their industrial complex was leveled due to the bombings.

The US had the land and the fermentation tanks and equipment, with the help of British scientists, penicillin was rapidly produced.  The most interesting part of the account I read, was in their search for a penicillin strain they spent most of their efforts with soil screenings.  The big breakthrough came when the most productive strain was isolated on a rotten cantaloupe in the Peoria fruit market!


Mechanism:  penicillin binds to proteins in the cell wall making holes in the wall and killing the bacteria outright.

Resistance:  has been a problem since the introduction of penicillin in the 1940’s.

  • The binding sites can adapt and alter the bacteria cell wall
  • Decreasing affinity of the target site can be altered.

Bacteria can produce enzymes that inactivate penicillin.

Indications for use

Are highly effective against gram positive cocci and some gram-negative cocci. Little effect on GM- rods.

Most common use: syphilis, susceptible pharyngitis and endocarditis and dental infections.

  1. tetani, C. Perfringens, Listeria, Syphilis.


Allergic reactions: in 3 to 10 % (rash 4-8%)  anaphylaxis: (0.01% to 0.05%)

Rarely N/V with oral use

Patient Education: finish entire prescribed dose. Take the entire prescription to completely eradicate the infection.  Do not “save up” antibiotics for future infections.

Report any rash to practitioner

Most common Drugs & dosage of this class

Penicillin VK (PenVeeK® & V-Cillin-K®)

Available as tablets: 250mg & 500mg.  Liquid= 125mg/5ml & 250mg/5ml

Liquid is reconstituted by pharmacist. Good for 14 days after mixing. Does have a bitter taste after reconstitution.

Penicillin G (Pen G & Bicillin L-A)

Pen G is a natural penicillin that most commonly given intramuscularly (IM)

  • Pen G comes in two unique IM formulation; Benzathine Pen G and Procaine Pen G
  • These IM Repository formulations allow for steady release of medications at therapeutic

doses for extended intervals

  • Great option for kids that a practitioner is worried that a caregiver may forget doses or non-compliant patients
  • Most common formulation/use; Benzathine Pen G 2.4 million units IM as a single dose for syphilis 

Penicillinase resistant Penicillins  (“anti-staph penicillins”)

Examples: methicillin, oxacillin (1971), cloxacillin (1971) & dicloxacillin (1968).

Good for methicillin sensitive strep and staph.

Are not affected by enzymes that the bacteria produce.

Generally used for skin and soft tissue infection

Extended Spectrum Penicillins

Have a broad spectrum of coverage, but most importantly … Pseudomonas aeruginosa

Examples: Piperacillin/tazobactam (Zosyn) and Ticarcillin/clavulanate (Timentin)

Zosyn is the safer option that is most common used.  These drugs are almost always used in the hospital setting.

Taking your entire prescription of antibiotics is necessary to help you completely eradicate your infection, as well as helping stop antibiotic resistance.  Always make sure your Thompson Pharmacist and your physician have an up to date list of allergies as well. Got a question about antibiotics… Go Ahead and ASK…at Thompson Pharmacy It’s ALL for YOU!