Drugs may interact in three ways when taken together: synergism, where their combined effect is greater than the sum of their individual effects; additivity, where their effects simply add up; and antagonism, where one drug decreases or abolishes the effect of another. Synergism can occur through potentiation or when drug effects are additive. Antagonism can occur via physical, chemical, physiological, receptor-based, competitive, or irreversible mechanisms. Pharmacogenomics studies an individual's genetic basis for drug response to guide optimal treatment, while pharmacoeconomics compares costs and outcomes of different treatment options.