In the U.S., terbutaline (Brethine) is one of the most frequently used drugs in the treatment of preterm labor. Terbutaline is in the class of drugs called beta-mimetics. Terbutaline can be given in the following ways: orally (by mouth), by injection under the skin (also known as "sub-Q," which is short for subcutaneous), or intravenously (through an IV). The common oral dosage ranges from 10 to 30 mg, with a maximum daily dose of about 40 mg. The subcutaneous dose is 0.25 mg every 20 to 60 minutes until a woman's contractions have subsided.
A new form of giving terbutaline, the continuous subcutaneous infusion pump, allows terbutaline to be easily given around the clock without the need for an IV. There are, however, disadvantages: the pump is expensive and is more invasive than the oral or subcutaneous route. Also, because there have been only a few small studies on use of the pump, it's not clear just how effective it is.
Terbutaline is derived from a hormone called epinephrine, which is released when a woman is under stress (a response that's commonly called the "fight or flight" response). Stress causes many of the muscles in a woman's body to contract, so that she is ready to respond quickly. One type of muscle in the body (smooth muscle), however, relaxes when a woman is under stress. Since most of the uterus is made up of smooth muscle, the uterus will relax in response to a drug that contains substances like epinephrine.
Women respond differently to terbutaline, so its effects and how long they last vary from woman to woman. When a woman has a good response, terbutaline reduces the number and frequency of contractions. And yet, like all tocolytic drugs, terbutaline has not been shown to consistently prevent or delay preterm delivery for a significant period of time.
Even so, studies have shown that terbutaline can usually delay delivery for at least several days (depending on how much a woman's cervix is dilated before beginning medication). This isn't a lot of time, but it can make a big difference for the fetus if the mother is given steroids along with terbutaline. After 48 hours, steroids improve a baby's lung function and reduce his or her risk of dying by 40%.
For the Mother
Because terbutaline is related to hormones released in the "fight or flight" response, a woman may experience the same effects when taking terbutaline as she does when under stress. About one in three women will have a racing heartbeat, flushing, tremors, and restlessness. Although these side effects can be annoying, they're usually not serious.
A few women (1 to 5%) have more serious side effects, such as irregular heartbeats, extra fluid in the lungs (pulmonary edema), and chest pain. The more serious side effects tend to occur when women are taking high doses, but the effects can also occur with standard doses. Terbutaline can also increase a woman's short-term risk for diabetes, in addition to raising blood sugar levels in women who already have diabetes.
For the Baby
Terbutaline can cause a temporary increase in the baby's heart rate and blood sugar levels. These effects are not usually serious and are easy to treat after delivery, if they should occur.
Women who have medical conditions that could be made worse by the side effects described above should not be given beta-mimetic drugs. This includes women with heart disease, hyperthyroidism, and poorly controlled diabetes.
The FDA issued an advisory in February of 2011 regarding the use of terbutaline in the treatment of pre-term labor. This warning was specific to the “off-label” use of terbutaline to treat pre-term labor. The warning says not to use the oral form of the drug to treat pre-term labor because it does not work, and that injectable terbutaline should be used only in urgent situations and not for more than 48 to 72 hours. However, it must be noted that each patient and situation is different. It is important to be aware of this warning, but specific situations may result in this drug being used by specialists for longer periods under close supervision.