Chloroethynylnorgestrel

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Chloroethynylnorgestrel is a steroidal progestin in the 19-nortestosterone family, related to norgestrel. It was studied as an oral contraceptive in the 1970s under the developmental code WY-4355.

When tested in combination with mestranol, and similar to other progestogens like ethynerone and anagestone acetate, it caused striking mammary tumors in beagle dogs after long-term use at very high doses (about 10- to 25-fold higher than human doses). Because of these findings, its development was halted, along with some other progestins that were also removed from contraceptive markets.

Later research showed the cancer risk appeared to be species-specific to dogs and not present in humans. Chloroethynylnorgestrel was never marketed.