Medicines used to fight bacterial infections are called antibiotics. Those used to fight fungal infections are called antifungals, while those that fight viruses are antivirals. All of these drugs may be grouped under the term anti-infectives. However, in this discussion, the term antibiotics will be used more generally to refer to all three.

Penicillin (PenVK), tetracycline (Sumycin), and sulfa drugs (trimethoprim-sulfamethoxazole, Septra) are among the better-known types of antibiotics. Some antibiotics (such as penicillins) are narrow spectrum-that is, they attack a single or several specific infections. Broad spectrum antibiotics (tetracyclines or ampicillins) attack a range of bacterial illnesses.

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Some types of bacteria are naturally more resistant to antibiotics than others. This is true, for example, of gram-negative bacilli-such as Campylobacter, Salmonella, Shigella, and Vibrio. Unlike other types of bacteria, these have a double-membrane surrounding each cell, which partly explains their added toughness against antibiotics.

Though antibiotics are useful drugs, they should only be taken when necessary because:

  • antibiotics can cause harmful side effects, ranging from stomach upset to allergic reactions, birth defects, or even death. Also, while fighting infection-causing bacteria, antibiotics can kill some of the bacteria that benefit the body. This can hinder the body's ability to prevent and fight illness; and
  • antibiotics can become less effective over time. Overuse of antibiotics can actually strengthen bacteria and make them resistant to treatment. This is now a serious worldwide issue. The injudicious, improper, or unnecessary use of antibiotics over time has led to the development of more and more resistant bacteria, which are becoming harder and harder to treat.

If doctors prescribe antibiotics when they are not necessary, patients may be exposed to needless risk. This is especially true during pregnancy, because both the mother and her baby are exposed. Some drugs may be completely harmless to an unborn child, but others have been known to cause major malformations.

Because only a few controlled scientific studies have addressed whether drugs are safe to use during pregnancy, physicians usually rely on data from animal research and from the collective experience in practice to decide whether to prescribe antibiotics to a pregnant woman. In 1979, the Food and Drug Administration (FDA) developed a classification system for drugs, including anti-infectives, with regard to their potential for having harmful effects on an unborn child:

Category A
Controlled studies in women fail to demonstrate a risk to the fetus in the first trimester. There is no evidence of risk in later trimesters. The possibility of fetal harm appears remote.

Category B
Animal reproduction studies have not demonstrated a fetal risk, but there are no controlled studies in pregnant women. Or, animal reproduction studies have shown an adverse effect (other than a decrease in fertility), but which was not confirmed in controlled studies of women in the first trimester (and there is no evidence of risk in later trimesters).

Category C
Either studies in animals have revealed adverse effects on the fetus (causing abnormalities or death) and there are no controlled studies in women or studies in women and animals are not available. Drugs in this category should be given only if the potential benefit justifies potential risk to the fetus.

Category D
There is positive evidence of human fetal risk, but the benefits from the use in pregnant women may be acceptable despite the risk-for example, if the drug is needed in a life-threatening situation or for a serious disease for which safer drugs cannot be used or are ineffective.

Category X
Studies in animals or humans have demonstrated fetal abnormalities, there is evidence of fetal risk based on human experience, or both. The risk of use of the drug in pregnant women clearly outweighs any possible benefit. The drug should not be used by women who are or may become pregnant.

Here are a few general rules of thumb on using antibiotics during pregnancy:

  1. Since the majority of antibiotics have not been studied in controlled trials, most ?safe? antibiotics are classified as FDA Category B.
  2. In general, unborn babies are most likely to be harmed when they are most immature-when their organs and tissues are just developing (first trimester of pregnancy). One exception to this is the use of sulfa antibiotics, commonly used for urinary or other infections in combination with another antibiotic, trimethoprim, in the drug Septra or Bactrim. While Septra does not cause congenital abnormalities and is safe for use early in pregnancy, it can cause jaundice in newborns. It is generally not used later in pregnancy.
  3. It is important to remember that the choice of an antibiotic relies on multiple factors, including the targeted organism, the possibility for resistance, and the potential for adverse effect on pregnancy and lactation. Very few medications are absolutely contraindicated in any situation. Likewise, very few medications are universally appropriate. Your doctor should be able to explain his particular choice of antibiotic, and he should be able to help you balance the risks and benefits of its use.