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. Also, there is some conflicting information about whether dextromethorphan, the DM in some cough medicine names, may be linked to birth defects such as neural tube defects and cleft palate.
Do I need to use the brand Robitussin?
No. Robitussin is the most common brand name, at least in the U.S.