|>>|| No. 2969
There is no expectorant which works as intended. Nothing you can buy over a counter will work to speed up recovery from a cough.
The only reasons why drugs such as this are continued to be sold is partly because the use very careful wording (i.e. "thins and loosens"), and partly because most of the drugs sold as expectorants are older than many of the laws controlling the sale of medications.
All that Guaifenesin does is stimulate your lungs into producing more mucus, yes that's more. When you take a cough medicine such as this the effect is that your lungs produce lots of extra watery phlegm, you start coughing that up and it makes you think "look at all this extra phlegm I'm coughing up, it must be working".
Unfortunately as long as you keep taking the cough medicine, your lungs will continue producing lots of phlegm, and so you never actually recover as the medicine is causing the symptoms that they imply it's curing.
Levomenthol on the other hand, is just there to soothe your throat, it can actually cause you to cough less, and when you cough less you stop clearing as much mucus. Hence levomenthol can also cause the symptoms to linger for longer.