Would you recommend tamoxifen x 5 years following mastectomy in a male with pure DCIS?

I would not. The risk of developing contralateral breast cancer in a woman with DCIS is about 5% per decade, or 0.5% per year. At this level of risk, chemopreventive tamoxifen is an appropriate consideration. The risk of contralateral breast cancer in a man with DCIS is uncertain, but is almost certainly very much less than 0.5% per year. At this level of risk, I would not recommend chemoprevention with tamoxifen.

Obviously, this man should be evaluated for breast cancer risk factors, particularly breast cancer predisposition gene mutations. Even if a deleterious variant is identified, his risk for contralateral breast cancer would be too low to justify the low but not negligible toxicity of tamoxifen, in my opinion.