A review on the theory, instrumentation and analytical applications of surface- enchanced Raman spectroscopy (SERS) is presented. The versatility of the Raman effect for analytical purposes is emphasized. In the present work, He-Ne and argon ion lasers were used to obtain surface- enhanced Raman spectra of 26 drugs on colloidal silver. Eighteen drugs resulted SERS-active, providing good quality vibrational bands. The effect of preaggregation with an external electrolyte has been evaluated. The limits of detection ranged between 0.019 µg/ml for spironolactone and 0.5 µg/ml for propranolol. Different binary mixtures of drugs have been studied. The drugs were adsorbed selectively on the metal surface depending of their chemical structure.