Been there, done that. Had the recommended 5 years and am now on Femara (Letrozole.) Being active in the survivor community, I know at least 40 other people who took or are currently taking Tamoxifen. Tamoxifen has been studied for at least 20 years, with a lot of research and clinical trials. I have tons of info, but will start with some basics.

Indications: ER (estrogen receptive) positive tumors only. Tamoxifen is mostly ineffective on ER negative tumors. ER + or - is determined by testing a tissue sample taken during the biopsy or other surgery.

How it works (in fuzzy lay terms): Tamoxifen creates fake receptors so existing estrogen will bond to the fake ones as opposed to the real. Estrogen-based cancer can't thrive on the fake ones.

The standard # is, Tamoxifen will reduce the chance of recurrence by 50%. I was given 40% chance of recurrence; with Tamoxifen, this was reduced to 20%.

Risks: 1. slight increase in uterine cancer. Uterine cancer is always slow growing and can be contained by a hysterectomy. Twice a year, I saw a gynecologist who specialized in cancer patients.
2. blood clots, which are potentially fatal. Risk goes up with overweight and smoking. Blood clots linked to Tamoxifen have not been recorded in average weight women under 50 years old. I personally don't know anyone who got blood clots or uterine cancer on Tamoxifen.

Side effects, at least 75% probability by my unscientific observations: 1. weight gain. I know women who put on 50 lbs. in 6 months. I gained 9 lbs. in 2 months, and I was eating less. To put this in perspective, I had only gained 5 lbs. since high school graduation prior to that. The way I look at it, my food thermostat broke. 2. hot flashes and night sweats. The more overweight and inactive a woman is, the more likely these are. Lucky me never got a one.

What your sister needs to know is what her projected recurrence risk % is. That will help her assess her risk vs. benefit in taking this. If she's ER -, I'd say don't.