A woman came up to the counter yesterday to ask about Robitussin as fertility aid. I was at the other end of the counter doing God knows what, but got called over when the pharmacist I was working with didn’t know the answer. Her friend had told her that Robitussin could help her conceive, and she had “read on the Internet” — a statement that always makes me cringe — that this was indeed possible. (This is probably the page she read, btw.)
But she couldn’t remember which type of Robitussin her friend told get, and needed our help.
The pharmacist pulled an answer out of his ass and made it sound really good. Turns out it was the right one. Guaifenesin, of course, thins mucus and he suggested that it might also thin the cervical mucus, allowing sperm to more easily penetrate. Seems this is, in fact, the idea behind using it to aid in fertility.
There is a small body of published literature that supports its use. One article (PDF) from Fertility and Sterility, published in 1982 stands out:
Couples with infertility should not use vaginal lubricants, which can impair sperm motility and activity. Twenty-three out of 40 females taking guaifenesin (200 mg orally three times a day) from day 5 to the day of BBT rise demonstrated improved cervical mucus quality, and 15 out of 23 couples conceived.
A second study published in 1991 (PDF). These findings are only relevant in the case where cervical mucus is abnormal, and can’t be applied outside of this context. I’m not a fan of taking OTC or prescription meds willy-nilly, even if it is “only” Robitussin. Especially if the reason is because a friend told you so, or you “read it on the Internet”.
Talk to your doctor, naturally. But I thought it was pretty interesting. But talk about off-label usage! 
I’ve suggested that to people for years. My other postulated mechanism of action is enhancing ciliary motion in the fallopian tubes. After all, there are cilia in the respiratory tract also. Yes, I know it’s supposed to work by enhancing water secretion by goblet cells, hence thinning the mucus, but what the hell. It doesn’t do any harm, and it’s biologically plausible.
When it comes to fertility treatments, it can be frustrating to try
to find something that will work and doesn’t cost an arm and a leg.
Strictly speaking, there are no over the counter fertility drugs
available that will help to increase fertility. If you want a drug
treatment, you’ll have to get it through your doctor.
Not only are these treatments incredibly expensive, they are also
known to have some unpleasant side effects. Fortunately, there are some
herbal supplements that have shown a great deal of promise in helping
couples conceive naturally.