Fight Cough

For Dry cough – should use cough syrup which has written on it ‘Suppresant’.
For Wet cough - should use cough syrup which has written on it ‘Expectorant’.

The reason he said is Suppresant is basically for dry cough as there is no mucus and nothing to be thrown out of body. It reduces the urge to cough and reduces dry cough.
For wet cough , one should never take Suppresant cough syrup. Wet cough produces mucus and this needs to be thrown out. Use Expectorant cough syrup as that helps the body throw out. Not use Suppresant as that suppreses.

We should never use vice versa. I found this piece of info very useful and true.