Now, nearly a week later, the 19-year-old from Ft. Lupton, Colo., may lose her baby because of the mistake.
"I am kind of overwhelmed now," said Silva, who only just learned she was pregnant. "I don't feel like talking to anyone."
"I am so worried about myself and the baby right now," she said, crying in an interview with ABCNews.com. "They [the media] haven't stopped coming to my house."
The drug, methotrexate, is used for chemotherapy and has "very serious side effects," and "may harm the fetus," according to the National Center for Biotechnology. It is sometimes used for early-stage pregnancy termination.
"It couldn't be a worse mix up than that," said Dr. Paul Doering, professor emeritus of pharmacology at the College of Pharmacy University of Florida. "There is no excuse for them. I can't think of anything worse."
Silva went to the pharmacy because she had a bacterial infection. But when she got home, she realized there was a different name on the label.
"I just took one," she said. "But it was so much more of a big deal because I am only six weeks along and it's a very delicate time in pregnancy."
She said the pharmacist knew she was pregnant when he gave her the methotrexate. "He just asked me my name and stuff," said Silva. "The names were real similar."
"He helped my pick out my prenatal vitamins and even told me congratulations," she said. "I had just barely found out I was pregnant on the second [of February]."
Doering, who did his master's thesis on the drug, said methotrexate is the "poster child for why certain medicines shouldn't be used in pregnancy. It's such a well-known and potent cause of birth defects."
In some instances taking the drug during pregnancy warrants termination, according to Doering.
"There are downsides to that and risks," he said. "But if it were my wife or daughter, I would recommend it."
Mix-ups like this are rare, but when they happen, they can be fatal.
Some studies have shown more than 26 to 30 deaths in recent years due to methotrexate dosage errors, according to Mike Cohen, president of the Institute for Safe Medicine Practice, which operates the national medication reporting program.
"There is no question, we've have problems with that drug," said Cohen. "Unfortunately, people do have serious reactions to it and it can be fatal."
Methotrexate, which is also used to treat psoriasis and rheumatoid arthritis, is a dangerous drug, generally to be taken only once week. Antibiotics are given several times a day.
"Doctors accidentally misprescribed or the pharmacists typed in one tablet daily," said Cohen. "It's given once a week or twice a week at most."
The prescription was filled at the Ft. Lupton Safeway and was intended for a 59-year-old woman with the same last name and a similar first name. Safeway apologized for the mix-up, but now Silva's pregnancy may be at risk.
Doctors say that because Silva was so early in her pregnancy, the fetus is the most vulnerable.
"This is my first child, so it's really difficult to deal with," said the expectant mother.
Silva, said she began to feel nauseated after taking the pill.
"My doctor immediately told me to try and make myself vomit to see if I could get the medicine to come back up," she said. She was then rushed to Platte Valley Medical Center where she was administered charcoal to absorb the methotrexate.
"For all this to happen now is really overwhelming, to know that I have to come home and wait," said Silva.
It's hard to know how often pharmacies mix up drugs, according to Dr. Alasdair Conn, chief of emergency services at Massachusetts General Hospital.
"The inquiry has not yet been performed but I would not expect a member of the public to question a prescription that has been filled and is labeled 'amoxicillin,' for example," he said. "The patient would have to look at the pills and use a picture dictionary to make sure that the pills are truly amoxicillin-- all tablets and pills have identifiers on them. But I think it is a stretch for anyone to check up in a filled prescription."
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