Med Headlines - Oh, no. Here they come – nausea, hot flashes, night sweats. These are symptoms no one enjoys and no one wants to face five years dealing with them. However, a new study by a team of researchers from the Mayo Clinic and the University of Michigan, seems to indicate these uncomfortable symptoms can be a very good sign for some women taking Tamoxifen to combat the recurrence of breast cancer.

The study focuses on the presence of an enzyme, CYP2D6, that activates the beneficial effect of Tamoxifen, a drug frequently used to minimize the risks of a certain type of breast cancer and the recurrence of it after it’s been treated.

When breast cancer cells have receptors for the hormone estrogen, Tamoxifen is an effective treatment. These estrogen receptor positive breast cancers account for about 75% of all forms of breast cancer.

Some women, about 10%, are genetically predisposed to produce very low levels of CYP2D6, meaning it will be unlikely that Tamoxifen will be an effective treatment for these patients. Alternative treatments are likely to be better.

After initial treatment for estrogen receptor positive breast cancer, a five-year course of Tamoxifen is recommended.

Unfortunately, many women stop taking Tamoxifen before the five-year therapy is completed due to its unpleasant side effects – night sweats, hot flashes, and nausea.

This study finds that women who have high levels of CYP2D6, which metabolizes Tamoxifen, are more likely to suffer these side effects. In fact, the higher the CYP2D6 level, the stronger the side effects.

These side effects are almost nonexistent in women with low or no CYP2D6 levels. These are also the women most likely to continue the entire course of treatment.

These women, too, are the least likely to benefit from Tamoxifen since the absence of CYP2D6 means their bodies are unable to use the drug.

With the case of Tamoxifen and CYP2D6, the side effects are actually a sign that the drug is working and should be continued for the full course of treatment. Those with no side effects should switch to a different therapy.

Genetic testing for the presence of CYP2D6 at the beginning of treatment can determine who is better off taking Tamoxifen and who would fare better with another drug. The genetic test, however, costs about $500, making it cost prohibitive in many cases.