Table 1.

Main pharmacological features of drug resistance versus variability in drug response.

Resistance	Variability
(i) is usually triggered by drug exposure, slowly reversible, and often drug specific	(i) not induced by and often independent of the drug, can affect different drugs, and does not revert upon withdrawal
(ii) implies a change of the drug target making it inaccessible or no longer inhibitable	(ii) the drug target is not necessarily modified or inaccessible
(iii) detectable by specific laboratory tests, which impact on the clinical decision of changing drug	(iii) laboratory tests are of little help in therapeutic decisions if mechanism(s) are unknown (change drug, increase dose, more frequent intake?)