Penicillin Antibiotics (Amoxil, Trimox)

Brand Name - Amoxicillin, Amoxil, Trimox

  • Prescribed for: Bacterial and other infections susceptible to the individual antibiotic.

Penicillin Antibiotics (Amoxil, Trimox) General Information

Penicillin-type antibiotics fight infection by killing bacteria and other micro-organisms. They do this by destroying the cell walls of the invading organisms. Other antibiotics simply prevent the invading organisms from reproducing. Many infections can be treated with almost any kind of Penicillin, but some can be treated only by a specific Penicillin antibiotic.

Penicillin cannot cure a cold, flu, or any other viral infection, and should never be taken unless prescribed by a doctor for a specific illness. Always take your antibiotic exactly according to your doctor’s directions, including the number of pills to take every day and the number of days to take the medicine. If you do not follow directions, you will not get the antibiotic’s full benefit.

Cautions and Warnings

Serious and sometimes fatal allergic reactions have occurred with Penicillin. Although this is more common following injection of the drug, it has occurred with Penicillin taken by mouth and is more common among people with a history of sensitivity to this or another Penicillin antibiotic or those who suffer from multiple allergies. About 5 percent of people who are allergic to a Penicillin antibiotic will also be allergic to the cephalosporins.

Some Penicillin drug reactions can be treated with antihistamines and other medicines. In a small number of cases where the infection is life threatening and cap be treated only with Penicillin, minor reactions may be treated with other medicines while the Penicillin is continued. Generally, though, other drugs can be substituted to treat the infection.

Cystic fibrosis patients are more likely to suffer from drug side effects to some Penicillins.

Penicillin Antibiotics Possible Side Effects

The most important Penicillin side effect, seen in up to 10 percent of people who take these antibiotics, is drug allergy. These reactions are more common among people who have had a previous reaction to Penicillin and those who have had asthma, hay fever, or other allergies. Some of the allergic symptoms include itching, rash, swelling, breathing difficulty, very low blood pressure, blood vessel collapse, skin peeling, and other severe reactions, including chills, fever, muscle aches, arthritis-like pains, and feelings of ill health.

About 9 percent of Ampicillin users develop an itching rash, which is not a true allergic reaction. This is more common if they are also taking Allopurinol (15 to 20 percent).

  • Common: upset stomach, abdominal pain, nausea, vomiting, diarrhea, colitis, sore mouth, coated tongue, anemias, bleeding abnormalities, low platelet and white blood cell counts, and oral or rectal fungal infections.
  • Less common: vaginal irritation, appetite loss, itchy eyes, and feelings of body warmth.
  • Rare: yellowing of the skin or eyes.
    People who receive Injectable Penicillin may become lethargic, dizzy, or tired, or may experience hallucinations, seizures, anxiety, depression, confusion, agitation, or hyperactivity.

Penicillin Antibiotics Drug Interactions

  • Penicillin should not be given together with a bacteriostatic antibiotic such as Chloramphenicol, Erythromycin, Tetracycline, or Neomycin, which may diminish the effectiveness of Penicillin.
  • Penicillin may interfere with the effectiveness of oral contraceptive drugs.
  • Penicillin allergic reactions may be intensified by beta-blocking drugs.
  • Ampicillin may reduce the effect of Atenolol by interfering with its absorption into the blood.
  • Large doses of injectable Penicillin can increase the effect of anticoagulant (blood-thinning) drugs.

Food Interactions

Do not take Penicillin with fruit juice or carbonated beverages, because the acid in these beverages can destroy the drug. Most of the Penicillins, including Bacampicillin suspension, are best absorbed on an empty stomach. These medications can be taken 1 hour before or 2 hours after meals, or first thing in the morning and last thing at night with the other doses spaced evenly through the day.

Amoxicillin, Amoxicillin and Potassium Clavulanate, and Bacampicillin tablets can be taken without regard to food.

Penicillin Antibiotics Usual Dose


  • Adult: 250 to 500 mg every 8 hours.
  • Child: 10 to 20 mg per pound per day, divided into 3 doses.

Amoxicillin and Potassium Clavulanate

  • Adult: a “250” or “500” tablet every 8 hours.
  • Child: 10 to 20 mg per pound per day divided into 3 doses.


  • Adult: 1 to 12 grams daily, divided into 4 to 6 doses.
  • Child: 25 to 100 mg per pound per day, divided into 4 to 6 doses.

Ampicillin with Probenecid

3.5 grams of Ampicillin and 1 gram of Probenecid as a single dose for gonorrhea.


  • Adult: 400 to 800 mg every 12 hours; 1,600 mg plus 1 gram of Probenecid for gonorrhea.
  • Child: 12 to 25 mg per pound per day, divided into 2 doses.

Carbenicillin Indanyl Sodium

  • Adult: 382 to 764 mg 4 times per day.
  • Child: not recommended.

Cloxacillin Sodium

  • Adult: 250 mg every 6 hours.
  • Child: 25 mg per pound per day, divided into 4 doses.

Dicloxacillin Sodium

  • Adult: 125 to 250 mg every 6 hours.
  • Child: 6 to 12 mg per pound per day, divided into 4 doses.


  • Adult: 250 to 1,000 mg every 4 to 6 hours.
  • Child: 5 to 25 mg per pound every 6 to 8 hours.


  • Adult: 500 to 1,000 mg every 4 to 6 hours.
  • Child: 25 to 50 mg per pound per day, divided into 4 or 6 doses.

Penicillin G

  • Adult: 200,000 to 800,000 units (125 to 500 mg) every 6 to 8 hours for 10 days.
  • Child (under age 12): 12,000 to 40,000 units per pound per day, divided into 3 to 6 doses.

Penicillin V

  • Adult: 125 to 500 mg 4 times per day. People with severe kidney disease should not take more than 250 mg every 6 hours.
  • Child (under age 12): 12 to 25 mg per pound per day, divided into 3 or 4 doses.


Penicillin overdose is unlikely, but if it occurs, diarrhea and upset stomach are the primary symptoms. Massive overdose can result in seizures or excitability. Call your local poison control center or emergency room for more information. ALWAYS bring the medicine bottle with you if you go for treatment.

Penicillin Antibiotics Special Information

Oral Penicillin liquids should be refrigerated (the bottle should be labeled to that effect) and must be discarded after 14 days in the refrigerator or 7 days at room temperature.

Call your doctor if you develop black tongue, skin rash, itching, hives, diarrhea, wheezing, breathing difficulty, sore throat, nausea, vomiting, fever, swollen joints, unusual bleeding or bruising, or feelings of ill health.

It takes 7 to 10 days for Penicillin to eradicate most susceptible organisms; be sure to take all the medicine prescribed for the full period prescribed. It is best taken at evenly spaced intervals throughout the entire day.

If you miss a dose of a Penicillin antibiotic, take it as soon as possible. If it is almost time for your next dose, space the missed dose and your next dose by 2 to 4 hours and then continue with your regular schedule.

Special Populations

Penicillin has not caused birth defects and is often prescribed for pregnant women. These drugs cross into fetal blood circulation and should be used only if absolutely necessary. Make sure your doctor knows you are taking a Penicillin antibiotic if you are or might become pregnant.
Penicillin is generally safe during breast-feeding; however, small amounts may pass into breast milk and cause upset stomach, diarrhea, allergic reactions, or other problems in the nursing infant.

Seniors may take Penicillin without special restriction.