It has been generally accepted within the clinical aromatherapy community that the use of essential oils containing estrogen-like phytochemicals or having "hormone balancing" properties is contraindicated for patients with hormone-positive reproductive cancers (Buckle 1997). Geranium falls under the category of "estrogen balancer", although to my knowledge, the exact mechanism by which this oil affects the pituitary/adrenal/ovarian axis is unknown.
I have always considered geranium to be a gentle and fairly benign oil with amazing balancing properties in terms of the endocrine, nervous and integumentary systems. I personally categorize geranium as an adaptogen, although I have never seen any specific reference to this moniker in any aromatherapy literature, scientific or folkloric. However, per the prevailing wisdom, I have always avoided using this oil on patients who have reproductive cancers.
Now it seems that my hypothesis on geranium may be somewhat correct. Recent studies have indicated that geranium essential oil actually has a positive interaction with tamoxifen, a commonly used breast cancer drug (Harris 2008).
Since I am currently working with two breast cancer clients who are on tamoxifen, I am happy to have an additional and very effective essential oil to add to my healing arsenal.
Interestingly, geranium oil has also been found to have a synergistic effect on some colon cancer drugs, increasing cytotoxicty of these drugs (Harris 2008).
Harris, Rhiannon. (17 Dec. 2008). Drug-Essential Oil Interactions: Risks and Reassurances. Presentation to Alliance of International Aromatherapists. Denver, CO.
Buckle, Jane. (1997). Clinical Aromatherapy in Nursing, 223. London: Arnold.