Penicillin – Allergy,Measurement,Dosage,Reaction


Penicillin – Allergy,Measurement,Dosage,Reaction.

Penicillin is one of the most useful antibiotics. It fights certain kinds of infections including many that produces pus. It does no good for diarrhea, most urinary infections, backaches, bruises, common cold, chickenpox or other virus infections.


Penicillin is measured in milligrams (mg) or units (U). For penicillin G, 250 mg = 400,000 U


For most people, penicillin is one of the safest medicines. Too much does no harm but only wastes money. Too little does not completely stop the infection and may make the bacteria resistant or difficult to kill.

Then for some people, penicillin causes allergic reactions. Mild allergic reactions include itchy spots or rashes. Often these come several hours or days after taken penicillin and may last for days. Antihistamines help to calm the itching.

Penicillin rarely causes a dangerous reaction which is known as allergic shock. After sometime penicillin is injected or swallowed, the person will suddenly get pale and has trouble breathing, then goes into the state of shock. Epinephrine (adrenalin) must be injected at once.

Make sure that you will always have epinephrine ready whenever you want to inject penicillin.

A person who has once had any allergic reaction to penicillin should never be given any kind of penicillin or ampicillin again; either by mouth or by injection. This is because the next time the reaction would likely be far worse and might kill the person. Stomach upset is not an allergic reaction when taking penicillin.

Persons who cannot take penicillin can sometimes be treated with tetracycline or erythromycin by mouth.

Most infections that can be treated with penicillin can be treated quite well with penicillin taken by mouth. Injected form of penicillin are more dangerous than those taken by mouth.

  NOTE : Use injectable penicillin only for severe or dangerous infections.



Sometimes penicillin does not work against an infection, it would normally control. This may be because the bacteria have become resistant that is penicillin no longer harm them.

Infections that are at times resistant to penicillin are

  • Impetigo
  • Sores on the skin with pus
  • Infections of the bone (osteomyelitis)

If one of the above infections does not respond to ordinary penicillin, another antibiotic may be tried or special forms of penicillin like methicillin, nafcillin, oxacillin, cloxacillin and dicloxacillin may work. In many parts of the world, gonorrhea is now resistant to penicillin. Also pneumonia is also sometimes resistant to penicillin but you can try co-trimoxazole or erythromycin.



Penicillin G or Penicillin V often comes in 250 mg (400,000 U) tablets. For suspensions or powders for suspension have 125 or 250 mg per teaspoon.

Penicillin V is used by the body more easily than Penicillin G but is more expensive. Penicillin by mouth rather than injection s should be used for mild and moderately severe infections which includes :

  • Abscessed or infected teeth
  • Infected wounds or many infected sores
  • Widespread impetigo
  • Ear infections
  • Sinustis
  • Sore throat with sudden high fever
  • strep throat
  • Some cases of bronchitis
  • Prevention of tetanus in persons who have not been vaccinated and who have deep or dirty wounds.
  • Rheumatic fever
  • Pneumonia

If infection is severe, it may be best to start with injections of penicillin but often penicillin by mouth can be given instead, once improvement begins. But if improvement does not begin within 2 to 3 days, you should consider using another antibiotic and try to reach out for medical advice.



Using tablets of 250 mg (20 to 60 mg./kg/day)



ADULT : 1 to 2 tablets (250 to 500 mg) = 4 times a day.

CHILDREN : 7 to 12 years (1 tablet 250 mg) = 4 times a day.

CHILDREN : 2 to 6 years (half tablet 125 mg) = 3 or 4 times a day.

CHILDREN : Under 2 years (quarter tablet ¼  62mg) = 3 or 4 times a day.

For more serious infections, double the above dosage.

Keep taking the penicillin for at least 5 days and for 2 or 3 days after fever and other signs of infection are gone. To help the body make better use of the medicine, an hour before meals (this is more important for penicillin G than for penicillin V).



Injectable penicillin should be used for certain severe infections which includes :

  • Meningitis
  • Septicemia (bacteria in the blood)
  • Tetanus
  • Severe pneumonia
  • Badly infected wounds
  • Gangrene
  • Infected bones and to prevent infection when a bone pokes through the skin
  • Gonorrhea
  • Syphilis
  • Pelvic inflammatory disease


Injectable penicillin comes in many different preparations. Before you inject any penicillin, be sure to check the amount of the kind.




Some of these penicillin do their work but does not last long. Then others work more slowly but last longer. There are times when it is better to use one kind than another.

Short-acting penicillin : These are known by many names including :

  • Crystalline penicillin
  • Benzylpenicillin
  • Aqueous penicillin
  • Soluble penicillin
  • Sodium penicillin
  • Potassium penicillin
  • Penicillin G infections

These penicillin listed above act quickly but only stay in the body a short time, so they must be injected every 6 hours (4 times a day). A short acting penicillin is the best choice for very severe infections when high doses of penicillin are needed. For example, for gas gangrene or when a broken bone pokes through the skin or meningitis.


Intermediate-acting penicillin :

Procaine penicillin or procaine penicillin aluminium monostearate (PAM).

These work more slowly and last about a day in the body, so injections should be given once daily. Procaine penicillin or a combination of procaine and a short acting penicillin, is the best choice for most infections when injectable penicillin is needed.


Long acting-penicillin :

  Benzathine or benethemine penicillin. This penicillin goes into the blood slowly and lasts up to a month. Its main use is in the treatment of strep throat, syphilis and for prevention of rheumatic fever. It is useful when a person lives far away from someone who injects or cannot be counted upon to take penicillin by mouth. For mild infections, a single injection may be enough. Benzathine penicillin often comes combined with faster-acting penicillin.


Crystalline penicillin (a short penicillin) :

They often comes in : vials of 1 million U (625 mg) or 5 million U (3125 mg)

Dosage of crystalline penicillin or any short-acting penicillin for severe infections

  • Give an injection every 4 to 6 hours
  • For adults and children over 8 years : 1 million U
  • Children 3 to 8 years : 500,000 U
  • Children under 3 years : 250,000 U

For meningitis and some other very severe infections, higher doses should be given


Procaine penicillin (intermediate-acting)

This often comes in : vials of 300,000 U, 400,000 U and more.

Dosage of procaine penicillin for moderately severe infections

  • Give one injection a day
  • Adults : 600,000 U to 1,200,000 U
  • Children 8 to 12 years : 600,000 U
  • Children 3 to 7 years : 300,000 U
  • New born babies : Do not use unless no other penicillin or ampicillin is available; in emergency 75,000 U

For very severe infections, give twice the above dose. However, it is better to use a short-acting penicillin. The dosage for procaine penicillin combined with a short-acting penicillin is the same as for procaine penicillin alone.

For treatment of gonorrhea that is not resistant to penicillin, procaine penicillin is best. Very high doses are needed. For pelvic inflammatory disease, the dosages are the same as for gonorrhea.


Benzathine penicillin (long-action)

This often come in : Vials of 1,200,000 U or 2,400,000 U

Dosage of benzathine penicillin for mild to moderately severe infections :

  • Give one injection every 4 days for mild infection
  • One injection may be enough
  • Adults : 1,200,000 U to 2,400,000 U
  • Children age 8 to 12 : 900,000 U
  • Children age 1 to 7 : 300,000 U to 600,000 U

For strep throat, give one injection of the above dose.

To prevent return infection in persons who have had rheumatic fever, give the above dose every 4 weeks. For treatment of syphilis, benzathine penicillin is best


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