Clinical symptomatology and behavior A. General principles A wide variety of drugs are classified as psychoactive because they primarily influence the way that people think, feel and behave. Small doses of most drugs produce highly variable and typically slight effects. Higher doses or chronic use typically produce more profound effects that are more predictable. Other routes of administration take longer for drugs to take effect.
Most drug effects diminish gradually over time as the drug is excreted from the body.
Most drugs can be detected for a day or two in the blood and for three days or longer in urine. A recently developed method for detecting drugs in hair can confirm use for up to six months after last use. Toxicological evidence of drugs in blood or urine cannot be used to determine with any degree of precision how large a dose was ingested, how long ago it was ingested, or whether the person was under the influence of the drug at a particular point in time. Once tolerance is developed, a person must take a higher dose to produce the same effects that were previously experienced with lower doses. Tolerance develops at different rates for different drug effects.