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Oral hormone replacement th erapy, or H.R.T., increases the risk for potentially fatal blood clots. But a study in BMJ found that hormone replacement delivered through the skin by injection or skin patch entails no increased risk for blood clots.

British researchers looked at 80,396 women who had blood clots while on various H.R.T. regimens, comparing them with 391,494 controls. After adjusting for ethnicity, smoking, alcohol use, chronic medical conditions and other factors, they found that oral preparations increased the overall risk of clots by 43 percent. Oral drugs containing equine estrogen more than doubled the risk in some regimens, although the absolute risk for blood clots is small.

But skin treatments, including patches, creams, gels and under-the-skin injections, had no effect on the risk for blood clots.

About 80 percent of the women in the study using H.R.T. took the medicine orally, and among the women who used transdermal regimens, 87 percent used the skin patches.