Alternative cough medicine
I won't repeat what everyone has already said about placebos and untested medicine. The thing is, though, there isn't really any evidence that the active ingredients of conventional cough medicine—such as dextromorphan—are effective either. The whole cough-medicine industry is essentially built on products that don't work.
Say what you like about morphine, but it will stop you coughing. :-)
(To add to the debate, I was once prescribed dihydrocodeine for a back injury and can confirm the constipation was shocking. Marvellous stuff though.)