Hey C,

I think the ingredient is guaifenesin. Just get the plain one... I'm not sure what it looks like as I haven't tried it yet, but this is something I found on the net...

What kind of Robitussin is used to improve cervical mucus?

One should buy the plain kind with no letters after it. The only active ingredient is guaifenesin, and it contains 100 mg per teaspoon. It is very important to avoid the versions that contain decongestants as those may dry up cervical mucus.
Good luck.