I used to find the mefenamic acid affected my stomach.
From the guardian.co.uk
In the past year I have started using pycnogenol supplements and it has reduced my pain significantly, to the point that I now need only mefenamic acid for 1-2 days at the most.
From the guardian.co.uk
I am in my mid40s and have had severe period pain for all my adult life and used to take mefenamic acid alternating with panadeine for about 5-6 days every month with the resulting upset stomachs.