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AURORA, COLO. (AP) -- Doctors say the early stages of pregnancy are an especially bad time for a pharmacological mix-up where a woman prescribed antibiotics instead is given a powerful drug used in chemotherapy.

Yet a pharmacist at a Colorado Safeway supermarket mistakenly gave Mareena Silva, who is six weeks pregnant, the drug methotrexate last week instead of the antibiotics.

Pharmacists say methotrexate targets the growth of rapidly dividing cells, such as those found in cancer tumors, but in cases of babies developing inside their mother, the drug can't tell the difference. Doctors say the drug taken during the crucial six to eight weeks of pregnancy could affect the development of the heart, the brain, the limbs and cranial facial features.

"These agents don't discriminate between a normal cell dividing or a cancer cell dividing," said Cindy O'Bryant, an associate professor at the University of Colorado School of Pharmacy in Aurora in suburban Denver. "It would not be able to tell the difference of a normal cell, a normal embryo cell, versus a tumor cell and because an embryo is rapidly dividing you're going to see more of an effect there because that's where the drug works."

The drug given out at the store in Fort Lupton, about 35 miles north of Denver, was intended for another woman with a similar name. It is commonly used for psoriasis and rheumatoid arthritis, in addition to cancer.

Silva told KMGH-TV in Denver that she took a pill and checked the bottle after becoming sick. She was rushed to the hospital.

"For all this to happen now, it's really overwhelming to know that I have to come home and sit and wait," Silva said in an interview with the station.

Safeway issued a statement that said policies and procedures meant to prevent medication errors were not adhered to, and that the company is redoubling efforts to ensure they are followed.

Those procedures include asking twice for the patient's full name and date of birth before handing out medication.

"We have extended our sincere apologies to Ms. Silva and offered to pay any medical expenses incurred as a result of the prescription error," Safeway's statement said. "We understand the anxiety this has caused and the difficulty of Ms. Silva's situation."

Silva told KMGH that the apology didn't change anything.

"Sorry is not going to cut it. I'm going to have to deal with this a long time," she said.

Methotrexate is one of the earliest chemotherapy drugs, O'Bryant and University of Florida College of Pharmacy professor Paul Doering said. Because of its ability to stop cell division, Doering said it was used by some doctors to perform abortions in the 1950s, but that practice was discontinued because it was ineffective and left fetuses with birth defects.

It can now be used to end so-called ectopic pregnancies, those that occur outside the womb, or in combination with another drug to induce abortion.

Dr. Julie Scott, assistant professor of obstetrics and gynecology at the University of Colorado School of Medicine and a specialist in high risk pregnancies said the critical time for methotrexate exposure is at between six and eight weeks of pregnancy.

"Which is when a lot of organ development is occurring," she said. "Specifically the brain, the heart, the limbs, some of the cranial facial features of the baby, so there could potentially be a specific impact. But with medications it becomes a dose related problem."

A paper based on cases studies on methotrexate exposure published in 1993 in the medical journal Teratology suggests that a reoccurring dosage of 10 mg per week or more could produce harmful effects.

"There are no randomized studies where we would actually submit a pregnant mom to this medication but these are reports of women who have been inadvertently exposed to medicines and then we collect the data based on studies," Scott said.

Scott said at this point, it would be up to doctors to closely monitor the development of the baby to see if exposure has caused any ill effects.

A woman who answered the door at Silva's apartment Tuesday said she had moved out and wouldn't talk to the media anymore based on the advice of an attorney. A message left with the woman for Silva was not immediately returned.

Fort Lupton is a small agricultural community on the eastern plains of Colorado, about 35 miles north of Denver.

(Copyright ©2011 by The Associated Press. All Rights Reserved.)