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There are medications that can be used, though they do carry some risk. Progestin and Mibolerone both works well. These medication should not be used to suppress more than two heat cycles in a row so this is not really a good long term idea.
However, these medications carry increased risk of cystic endometrial hyperplasia, uterine infection and mammary tumors. Diabetes, adrenal gland dysfunction and hair loss have also occurred.
There are no long term medications or tricks other then having her spayed. As far as the mess........consider picking up dog diapers or Belly Bands for dogs that can help prevent the mess. She will go into heat every 6 months. Most dogs stop going into heat at around 10 years of age but this can sometimes occur earlier in dogs that have never been mated. As for the upset stomach you can put her on a bland diet of Boiled chicken and rice to help. The general heat cycle is around 21-25 days.
Your very welcome.......I understand the issues with diapers. Try Belly Bands they work much better. You simply line them with a pantyliner and they look almost like womens undergarments.
You can find them on ebay and they are inexpensive and work perfectly.