H Hellen
I have also recently swapped from Vagifem to Ovestin. (Only used VF for a few days as it made me feel a lot worse due to the residue). I had initial stinging/ burning with Ovestin, then relief for a week or so, then soreness came back.
My GP had told me to use it every night for 4 weeks and then reduce to twice weekly. I found that the soreness returned before the 4 weeks were up though so she said I could carry on nightly for another 3 weeks only. I am now trying alternate nights while I wait for a GP app next week and I'm using moisturiser on the night I don't use Ovestin to see if that helps (I had stopped all moisturisers whilst using Ovestin because I was worried they might stop the Ovestin being absorbed).
Several people on here have said that Ovestin is ok to use nightly longer term though. I will report back what my GP tells me next week about worsening of symptoms again whilst still using every night and long term use. I wonder if sometimes it's to do with our own hormones (what's left of them!) going up and down, so sometimes the body needs more and we feel sore again, then it may settle again

I was also told something called the Estring can be helpful as it gives a steady, low dose of hormone if Ovestin isn't working for you.
