Physiological tolerance or drug tolerance is commonly encountered in pharmacology, when a subject’s reaction to a specific drug and concentration of the drug is progressively reduced, requiring an increase in concentration to achieve the desired effect. Drug tolerance can involve both psychological drug tolerance and physiological factors. The following are characteristics of drug tolerance: it is reversible, the rate depends on the particular drug, dosage and frequency of use, differential development occurs for different effects of the same drug. Physiological tolerance also occurs when an organism builds up a resistance to the effects of a substance after repeated exposure. This can occur with environmental substances, such as salt or pesticides. A rapid drug tolerance is termed tachyphylaxis.