(3) Natural penicillins are not affected by the metabolic process and are
excreted unchanged. Excretion is very rapid via the kidneys but can be retarded by the
concurrent administration of probenecid. Combining penicillin G with procaine or
benzathine may also retard excretion. This increases the amount of penicillin available
in the body, prolonging the action and increasing the effectiveness.
(4) Dosage of the natural penicillins depends on the dosage form and the
type and severity of the infection being treated. Parenteral doses are measured in
units, with one unit equal to 0.6 mg of standard penicillin, USP.
(5) The natural penicillins are narrow spectrum antibiotics primarily effective
against gram-positive and a few gram-negative bacteria. They are the first drugs of
choice in the treatment of infection caused by gram-positive cocci and bacilli, gram-
negative bacilli bacteroides, cocci, and spirochetes.
(6) The most common adverse reaction to these drugs is hypersensitivity.
The degradation product, penicillenic acid, probably causes this. The most common
manifestation is skin rash, with the most lethal reaction being anaphylactic shock. This
reaction may vary from a mild fever, rash, or leukopenia to severe arthralgia or arthritis.
Oral preparations may cause nausea and vomiting, epigastric distress, diarrhea, and
black hairy tongue. Superinfections may occur but are rare.
(7) The use of natural penicillin is contraindicated in patients with penicillin
allergy, which should be used with caution in patients with histories of other allergies.
When administered, especially parenterally, steps should be taken to ensure that
agents are available to manage hyper-sensitive reactions and to monitor the patient at
least 30 minute s after he receives a parenteral injection. The usual course of therapy
is 7-10 days, and the patient should be instructed to complete it.
The natural penicillins are found in the following preparations:
(a) K+ or Na+ penicillin G (Penicillin G).
(b) Procaine penicillin G (Wycillin, Crysticillin).
Benzathine penicillin G (Bicillin LA, Permapen).
(d) Penicillin V potassium (V-Cillin, Pen-Vee K).
b. Penicillinase Resistant Penicillins.
(1) Penicillinase is an enzyme produced by certain bacteria, which converts
penicillin to an inactive product and thus increases resistance to the drug. Drugs in this
group are structurally resistant to beta-lactamase activity, interfere with transpeptidases
of the cell, and are bactericidal.