Antibiotic Dosages

Dr. Greene's Answer

I can certainly understand your concern and confusion over the drug dosages given to your daughter. Medication errors can be a real problem. The issues you’ve raised aren’t restricted, though, to crossing national boundaries. I frequently hear the same concern from parents who have just gone across the street to our after-hours pediatric service or down the hall and seen another doctor in my own practice. Today, I met two adorable twin girls who had just moved here from Oregon. When I prescribed amoxicillin, I learned (when Mom asked about the dose) that the dose was different (less) than they had received before.

Many adult medications come in “one-size-fits-all” formulations. With most medicines for children, however, the dose varies with the child’s weight. Because the weights of children change a great deal with the years, these medicines often come in a variety of forms. Many of the liquid antibiotics come in two different concentrations. As you’ve noted, amoxicillin comes in 125 mg per teaspoon (5mL) or 250 mg per teaspoon (5mL). One teaspoon of the 125 mg strength is equal to one-half teaspoon of the 250 mg strength of the amoxicillin. (So in your case, your daughter got a slightly higher dose this time than previously.) The different concentrations make it easier to measure the appropriate amount to give to smaller children, while making sure that larger children don’t need to swallow too much liquid in order to get the correct dose.

In addition to the difference in strengths of the two formulations, there is a wide range of correct dosages for amoxicillin. The dose recommendations for children on amoxicillin range from 20 to 90 milligrams per kilogram of body weight per day . Thus two children could be on appropriate doses of amoxicillin, each weighing the same amount, and yet one is receiving over four times as much as the other.

Physicians will select where to dose a child, within the acceptable range, based on several factors:

• The individual history of the child
• The type of infection being treated
• The physician’s personal philosophy of antibiotic usage

As you might expect, smaller doses are not quite as powerful as the larger doses, but have the advantage of fewer side effects. If a child has had a history of problems with diarrhea or yeast infections, a physician might opt for a lower dose. If the child has had a history of infections that don’t clear up easily, but the child has not had much in the way of side effects, a physician might opt for the larger end of the dosing scale.

Also, different infections tend to be treated at different ends of the dosage spectrum due to the antibiotic’s ability to fight a particular infection. While strep throat can make children very sick, it is an easy infection to clear using the correct antibiotics. Amoxicillin kills strep throat readily, and a 20 mg per kg, per day dosage should be sufficient. With more stubborn infections, such as ear infections, the 40 mg per kg is a standard dose. But whenever resistant bacteria are suspected, high dose treatment with 60 to 100 mg per kg per day might be best. These high doses might also be important for serious bacterial infections.

Most scientific studies of amoxicillin’s effectiveness have used a regime of three times a day for ten days. While this remains the standard, some studies have shown advantages of three, five, or seven day courses. Also, there has been some success with once or twice a day dosing of amoxicillin (The Pediatric Clinics of North America, December 1996).

With all this to consider, there is a final point which is somewhat subjective — an individual physician’s personal approach to antibiotic use. Some physicians like to give the smallest amount of antibiotic that might work, in hopes of avoiding side effects, and maximizing the role of the child’s immune system in fighting the infection. Others physicians like to blast things out with the largest doses acceptable in hopes that they will actually use fewer antibiotics in the long run by killing the infection as quickly as possible.

As you can see, there are a wide range of options for amoxicillin that are all acceptable. You know your child’s past history better than any physician will ever know it (after all, you are the one to clean up after a bout of diarrhea!). You also may have an opinion on how aggressively you want to use antibiotics with your child. Because of this, it is a good idea to question your physician whenever there is a concern about your child’s drug doses. This is particularly true if the dosage has changed. Probably it will turn out to be appropriate, but if it doesn’t, you’ll be glad you asked.

N.B. Although I have used amoxicillin as the example in this answer, the same principles are true for other drugs. You are your child’s most profound advocate. It’s okay to question your physician any time you are not confident that your child is getting the best possible medical care available.