Questions surface over whether antibiotics neutralized the pill, resulting in pregnancy

July 1, 1996
A recent case involves a patient`s misfortune with an unwanted pregnancy. Speculation suggests that either or both of two separate courses of antibiotics reduced plasma levels of the oral contraceptive she was taking, resulting in pregnancy.

Cynthia R. Biron, RDH

A recent case involves a patient`s misfortune with an unwanted pregnancy. Speculation suggests that either or both of two separate courses of antibiotics reduced plasma levels of the oral contraceptive she was taking, resulting in pregnancy.

Janet Jones (a fictitious name, as are all others in this article), a 27-year-old female patient presents with pericornitis of tooth #1 (maxillary right third molar) on October 31, 1995. She has been a patient of record at Dr. Michaels for nine years. Her medical history has always been unremarkable.

During this emergency visit she is in much pain, and the dentist is hoping to alleviate the pain as soon as possible. In focusing on alleviating Janet`s pain, neither Dr. Michaels, his assistant, or the receptionist update Janet`s medical history by asking her if there are any changes or if she is taking any medication.

Medical history

If Janet`s medical history had been updated, they would have learned that she had a baby boy on January 10, 1995. The birth occurred without complications, and she has had no serious illnesses. She began taking an oral contraceptive (Nordette 28) on February 1, 1995.

Her vital signs were within the normal range, and she was a healthy young mother. She had been taking extra-strength Tylenol for the pain associated with tooth #1.

Dental history

Janet had not seen Dr. Michaels since February 1994. She had bitewing X-rays at that appointment, and her teeth were cleaned by the hygienist.

Janet has a full complement of teeth with very few one-surface restorations. Her oral hygiene is excellent, and periodontal tissues are healthy with the exception of occasional soreness and inflammation around the partially erupted third molars.

Dental treatment

To treat the pain, Dr. Michaels prescribed Tylenol 3. To treat the infection, he prescribed Trimox at 500mg. for three times a day for 10 days. Salt water rinses were recommended to Janet for reducing swelling and soreness. On November 6, 1995, Dr. Michaels extracted Janet`s tooth #1.

The extraction was simple, using xylocaine plain for local anesthesia. There were no dental post-op complications.

Medical treatment

On November 15, 1995, Janet saw Dr. Macy, her primary care physician. Her reason for seeing Dr. Macy was another infection - a sinus infection. Her symptoms included headaches, rhinitis, and congestion. Dr. Macy was the physician who had prescribed Nordette 28. He decided to prescribe an antibiotic for her sinus infection.

While writing out the prescription, he said to Janet, "I am giving you a prescription for an antibiotic for your sinus infection. You must use an alternate form of birth control while you are taking this antibiotic and throughout this cycle of taking the birth control pill. Although it is very rare, there have been reports that show that antibiotics can interfere with the effects of oral contraceptives."

To this Janet replied, "I saw my dentist last week to have an infected wisdom tooth pulled and he put me on antibiotics for 10 days and never told me to use an alternate form of birth control."

Dr. Macy said, "Well let`s hope you are not already pregnant. Be sure to use an alternate method of birth control now."

Janet had no adverse reactions to the antibiotics such as diarrhea or secondary opportunistic infections.

She did have an upset in her menstrual cycle even though she had been totally compliant with the oral contraceptive regimen. While taking the second course of antibiotics, she was 14 days into her cycle when she had a menstrual period of the type that she had when she was not on the oral contraceptive. While on the oral contraceptive her menstrual periods were scant and only lasted three days. This menstrual period that started 14 days early was heavier and lasted 5 days.

The very day Janet started this menstrual period she called Dr. Macy. He told her that she had probably ovulated and/or was pregnant. He instructed her to continue taking the oral contraceptive until the packet was finished. Janet did just that, and, when she finished the packet, she had another period like the previous one she had in the 14th day of her cycle. After the period, she took the oral contraceptive for the 21-day regimen, but when she was suppose to start her menstrual period she did not.

She purchased a home pregnancy test to determine if she might be pregnant. The test indicated that she tested positive for pregnancy. She contacted Dr. Macy, and he suggested she see her obstetrician for an examination and pregnancy test.

Dr. Hampton, the obstetrician, confirmed that she was pregnant and felt that the pregnancy occurred because antibiotics reduced plasma levels of the oral contraceptive.

Janet insists that after Dr. Macy informed her of the possible interactions of antibiotics that she and her husband had used condoms when they had sex for the remainder of her November cycle. She also insists she had not missed taking an oral contraceptive pill since she and her husband just could not afford another baby this soon.

She is due to have a baby on September 6, 1996. At least that is how the obstetrician has calculated her due date according to her last period. In the interim, Janet has had an ultrasound, and it appears as though she may be further along in her pregnancy than originally thought. She might be due on August 6, 1996, exactly nine months from the time she was on the antibiotic prescribed by Dr. Michaels, her dentist!

With all the significant variables of two courses of antibiotics, two menstrual periods after the antibiotics, and the 1 percent possibility of anyone on an oral contraceptive getting pregnant, it would be difficult to prove that the dentist`s prescription interfered with the oral contraceptive. The issue at hand is that Janet`s medical history was not updated, and the dentist did not stop to think that she might be taking oral contraceptives.

Or if he thought she might be, he wasn`t concerned, as the literature shows the risk of pregnancy in this situation to be very low. And there is still the possibility that the dentist was not knowledgeable of the fact that antibiotics could interfere with oral contraceptives.

Dr. Michaels has not commented on the case. Both the primary care physician and the obstetrician attribute the pregnancy to oral contraceptive failure from antibiotic interference.

How do antibiotics interfere with oral contraceptives?

The hormones in oral contraceptives work in different ways to prevent pregnancy. The estrogens suppress ovulation so that the egg is unavailable for permeation by sperm. The progestins alter endometrial tissues so that implantation cannot occur, and they cause cervical mucous to have such a high viscosity that it prevents sperm from entering the uterus.

Most oral contraceptives contain both estrogen and progestin in combination. The combination oral contraceptives prevent ovulation by suppressing the follicle stimulating hormone (FSH) and luteinizing hormone (LH). There are approximately three progestin-only oral contraceptives, and they act solely on cervical mucous and endometrial cellular structure.

Although the antibiotic mechanisms of interference with oral contraceptives are not thoroughly known or understood, it is believed that there are at least three means of interference:

- Antibiotics kill bacteria which aid in the breakdown and absorption of the drug conjugates causing a drop in plasma levels.

- An increase in urinary or fecal elimination causes the oral contraceptive to be eliminated before it can be absorbed. Antibiotics upset intestinal flora and frequently cause diarrhea. Other causes of diarrhea can also cause elimination of oral contraceptive and a drop in plasma levels of the contraceptive.

- Increased liver degradation of the contraceptive breaks the contraceptive down to ineffective products.

The antibiotics that have the highest incidence of interference with oral contraceptives are the penicillins, including amoxicillin, ampicillin, and several that are less likely to be prescribed in dentistry. The family of antibiotics that is second highest in incidence of interference with oral contraceptives are the tetracyclines. Other antibiotics used in dentistry that have been involved in oral contraceptive interference are sulfonamides, erythromycin, metronidazole, griseofulvin and cephalosporin.

What prevents interference with oral contraceptives?

An article in the May 1988 Fertility and Sterility reported that Syntex Laboratory of Research compiled all reports of drugs interfering with oral contraceptives. The statement from their findings was, "There were 713 reports involving 701 women. The data analysis showed that 21 percent of the interactions resulted in pregnancy, 41 percent resulted in menstrual disturbances, and 38 percent in no problems. 256 of the incidences involved antibiotics; anti-tuberculosis drugs accounted for 76 percent of these. The three other drugs most often indicated were anticonvulsants, antidepressants, and analgesics, and this group of four is associated with the most pregnancies reported." The most recent update in the Drug Facts and Comparisons of July 1994 states that the coadministration of antibiotics with oral contraceptives may decrease the pharmacological effects of the oral contraceptive.

The first one to educate the patient must be the physician who prescribes the oral contraceptive. Labels should be added to oral contraceptive prescription containers alerting patients of the risks of drug interference and the possibility of pregnancy. The next doctor to prescribe any drug, especially an antibiotic, to a woman of childbearing age must inform the patient of the likelihood that the drug being prescribed could interfere with oral contraceptives.

Pamphlets advising patients of drug interactions with oral contraceptives are available for dental offices. In some cases, teenage girls are reluctant to admit to their dentist that they are taking birth control pills. The pamphlets could be a means of communicating with these patients without embarrassing them. The pamphlets should be displayed in the reception area and handed to women of childbearing age along with antibiotic prescriptions

Documentation in the patient`s chart indicating that the pamphlet was given to the patient and that the patient was told to use an alternate method of birth control could be very valuable in the event of a legal situation.

Patients taking oral contraceptives are at risk for hypertension and should have their vital signs taken at recall appointments. If the blood pressure is elevated, it should be measured at each dental appointment before treatment. Because oral contraceptives are the most common method of birth control, some patients do not even think of it as a medication.

They may even forget to tell you they are on an oral contraceptive. So it is important to specifically ask this question when prescribing antibiotics, "Are you taking birth control pills?"

If they say they are not, include that notation in the patient`s chart after documenting the regimen for the antibiotic. A failure to inform a patient taking oral contraceptives during the antibiotic therapy could result in a lawsuit that would leave the dentist liable for child support. In this case with Janet, it is difficult to determine which antibiotic regimen could have interfered with oral contraceptive action or if a combination of the two different antibiotics is the cause.

The failure rate of oral contraceptives is 1 percent, and there is always the possibility that Janet is one of the 1 percent. In her case, she will provide a loving home for the new baby. In other cases, an unwanted pregnancy can mean an innocent child may be the victim. As health care professionals, all dental team professionals need to be made aware of oral contraceptive failure from medications that are prescribed in dentistry.

The "Medical Alert" column contains actual cases which either involved a need for a medical consultation, a management of a medical emergency, or a post-op complication. If you know of an interesting and informative case for our readers, please send a brief description of the case to: Cynthia R. Biron, Director of Dental Auxiliaries, Tallahassee Community College, 444 Appleyard Drive, Tallahassee, FL 32304-2895. Ms. Biron will contact you for a personal telephone interview. All cases will remain strictly confidential and names changed to provide confidentiality unless a dental team is in agreement with being giving recognition for a lifesaving medical situation.

Cynthia R. Biron, RDH, is chair of the dental hygiene program at the Tallahassee Community College. She is also a certified emergency medical technician.