Problems of drug resistance in tuberculosis--the newer anti-tuberculosis drugs.

The value of ethambutol and rifampicin is now established but certain points about toxicity and effectiveness are still uncertain, in particular the failure rate if these two drugs are used together without a third. Primary resistance to ethambutol has been described in the U.S.A. but seems rare in Britain. Acquired resistance to both drugs is now being seen in reference laboratories so that the drugs are already being wrongly used. It is suggested that, if at all possible, a third drug should be added to the rifampicin-ethambutol combination. The pros and cons of different drugs are discussed. In future an increasing number of relapsing cases will already have received the new drugs. The clinician will therefore have to work out for each patient the drugs to which his bacilli are probably sensitive. A reliable drug combination for that individual can then be devised. The principles of such an analysis are outlined and examples given.