While juggling the side effects of breast cancer treatment, I have heard this reasoning multiple times: The benefits greatly outweigh the risks. I’m not so sure the logic always works.

The next phase of my treatment includes taking a drug for five years that the American Cancer Society has deemed a human carcinogen, a substance that is directly involved in causing cancer. The drug is called Tamoxifen, sold as Nolvadex, Istubal and Valodex. The list of carcinogens also includes a leisurely day in Miami Beach: solar radiation, tobacco and alcoholic beverages.

“It is certain that if we put you on a medicine like Tamoxifen, we will dramatically further decrease your chance of recurrence,” said the University of Miami Sylvester Cancer Center’s Dr. Marc Lippman, chairman of the University of Miami Miller School of Medicine.

I trust Lippman. He is a pioneering breast cancer researcher, who understands the kind of tumors that were found in my left breast, which tested positive for estrogen and progesterone, the female hormones produced in the ovaries. The “anti-tumor” medicine he recommends is meant to fool the voracious appetite of the molecular monsters – which could be lurking in my chest wall.

“Tamoxifen is one of the safest drugs we have,” said Lippman. “It doesn’t cause any heart, lung, liver, kidney, brain, bonemarrow damage … believe it or not taking your Tamoxifen every day is worth as much to you than all the radiation and all the chemotherapy that you have had."

But the more I was learning about Tamoxifen, the more I felt like a scared tight ropewalker. The drug was born in an environment where patents allow legal monopolies to keep prices high and restrict competition. This environnment has often led to accusations that drug companies have withheld information about the risks of taking certain medications.

Imperial Chemical Industries, a former British chemical company, discovered Tamoxifen in 1978 and patented it in the U.S. in 1985. Seven years later, a district court judge declared the patent invalid based on the conclusion that ICI had deliberately withheld “crucial information’’ regarding tests that it had conducted on laboratory animals. The tests had revealed hormonal effects “opposite to those sought in humans,’’ which, the court found, could have “unpredictable and at times disastrous consequences.’’

ICI filed an appeal to the U.S. Court of Appeals, and in 1993, struck down the district court’s judgement that the patent was invalid. ICI eventually transferred the patent to pharmaceutical giant AstraZeneca, which now sells the widely prescribed drug. In recent cases involving another one of its products, AstraZeneca was accused of not disclosing studies that revealed serious risks. According to company e-mails unsealed in pending civil lawsuits against AstraZeneca, employees dealing with unfavorable studies engaged in doing a “great smoke-and-mirrors job” to market their product to doctors and patients.

During a press conference in 2010, Kathleen Sebelius, the secretary of health and human services, said AstraZeneca paid kickbacks to doctors as part of an illegal scheme to market drugs for unapproved uses.” AstraZeneca denied wrongdoing and paid $520 million to settle the federal investigations.