This section is from the book "Botanic Drugs Their Materia Medica, Pharmacology and Therapeutics", by Thomas S. Blair. Also available from Amazon: Botanic Drugs, Their Materia Medica, Pharmacology and Therapeutics.
Sundew, Drosera rotundifolia. Official in France and Mexico. The plant contains a peculiar acid whose character has not been defined, and an acrid resin. The drug has attained a reputation in the treatment of irritable and spasmodic cough, as whooping cough and the cough of measles. Give the fl. in doses of 5 to 30 minims.
I find it hard to estimate the real value of this drug or to determine its effective dosage. Dry and irritable cough is notoriously hard to relieve. We know that even codeine often fails. It has impressed me, after trial in a number of cases, that drosera is worthy of use in these cases. It often fails; so do other remedies recommended for irritable cough in which examination reveals no lesion capable of accounting for the persistent "barking" cough.