Caffeine interferes with drugs that regulate heart rhythm, such as quinidine and propranolol (Inderal).
Later quinidine and cinchonidine were discovered, and subsequently several other alkaloids, but in smaller quantity.
The other alkaloids are distinguished from quinine thus: quinidine resembles quinine, but is dextro-rotatory, and the iodide is very insoluble in water; the solution of cinchonidine, which is laevo-rotatory, does not give the thalleoquin test, nor fluorescence; cinchonine resembles cinchonidine in these respects, but is dextrorotatory.
The sulphate of quinine used in medicine may contain up to 3% of cinchonidine, but should be free from cinchonine, quinidine and cupreine.
The other alkaloids of cinchona bark - quinidine, cinchonidine, and cinchonine - also possess similar properties, but all are much less effective than quinine.