One recent morning, a 17-year-old college freshman was brought into the ER by ambulance. He was feeling weak and confused and had a dark purple rash and a fever. Christel, the registered nurse who first examined the teenager, quickly surmised that he might have a life-threatening infection, so she called the lab and told the staff there to expect a sample of spinal fluid (the best way to diagnose the illness she suspected). Meanwhile, she asked a colleague to draw blood while she started an IV line and paged me to come to the exam room to perform a spinal tap. Christel was right. The young man had neisseria meningitidis—a bacterial infection that causes meningitis and is well known to be fatal within 24 hours.

Because meningitis is so deadly and contagious, we needed to treat the staff who had close contact with the patient once he was admitted to intensive care. Most of the staff who treated him had no drug allergies or contraindications, so each of them was able to take just one tablet of the powerful antibiotic ciprofloxacin (Cipro). Christel, however, had several allergies. For that reason, her only choice was to take a two-day regimen of rifampin (Rifadin), an antibiotic that is used mainly to treat tuberculosis but also prevents meningitis. Rifampin is famous for turning one’s urine and tears red or purple, which is harmless unless you wear soft contact lenses. Like some antibiotics, it has also been shown to interact with literally hundreds of medications. All the hospital staff who treated the patient got complete instructions about the disease we were trying to prevent and the medication that was going to be given to protect them.

Fortunately, no one who cared for the patient got ill, and according to the college the young man attended, there were no further cases of the disease. However, about a month later, Christel pulled me aside. “Can rifampin change a woman’s menstrual cycle?” she asked. When I said that it can—because it can decrease the effectiveness of oral contraceptives—I could see Christel’s eyes open wide. “I better take a pregnancy test!” she said. As it turns out, she was indeed pregnant and admitted that she had been careless about the potential drug interaction. Eight months later, she and her husband welcomed a healthy baby boy to their family.

LESSON LEARNED: The use of antibiotics can be tricky. There are more significant side effects and interactions with these drugs than perhaps with any other medication. Diarrhea is a risk for anyone taking virtually any antibiotic. But other surprises include violent vomiting if you mix the antibiotic Flagyl with alcohol, and people using ciprofloxacin can suffer ruptured tendons. Not all doctors agree that all antibiotics have an effect on oral contraceptives, but rifampin has been clearly shown to make the Pill less effective. To be safe, I tell women who are taking a course of antibiotics that is 15 days or less to continue taking their contraceptives to avoid irregular periods—but to not trust the contraceptives for birth control for 28 days, which is one full menstrual cycle. This will virtually eliminate the chance of any obstetrical surprises!