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ADVANCES IN CONTRACEPTION. 1990 Sep; 6(3):169-76.Clinical trials of vaginal rings containing progestins or ethinyl estradiol and progestins by WHO, the Population Council and private firms are reviewed. Contraceptive steroids can be formulated into Silastic vaginal rings because they are released continuously from this material (zero-order kinetics). Vaginal rings have the advantage of avoiding the 1st pass effect on the liver, as well as self- administration, unrelated to the timing of coitus and regulation of withdrawal bleeding with removal for 7 days per cycle. The shell vaginal ring, with an inert core, a layer of Silastic containing the progestogen, and an outer Silastic layer is designed to regulate release by the thickness of the outer layer. The WHO tested rings releasing 200 mcg norethisterone/day resulting in too many menstrual side effects; and 50 mcg/day with too high a failure rate. A ring releasing 20 mcg levonorgestrel is expected to perform well. The Population Council designed rings releasing 152 mcg ethinyl estradiol and 252 mcg levonorgestrel, and 183 mcg ethinyl estradiol and 293 mcg levonorgestrel. These resulted in pregnancy rates of 2/100 woman years, and continuation rates of 50%, but unacceptably adverse lipid effects. Women discontinued for vaginal symptoms. Compared to a similar combined oral pill, the rings offered no advantage. WHO subsequently introduced a ring releasing 20 mcg levonorgestrel: efficacy was 3.8 and continuation over 50%. A new segmented ring with desogestrel is causing fewer androgenic effects and bleeding complaints. Another ring in current trials gives off 120 mcg desogestrel and 30 mcg ethinyl estradiol with no pregnancies and good acceptability in 100 women to date. Availability of Silastic material and quality control in manufacture are seen as obstacles to overcome for mass production of these vaginal rings.