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.
Thus, a person may experience or exhibit some but not all effects of a drug once they have become tolerant. Thus a person may take daily doses that would be fatal to a person who is not tolerant. The effects of drugs may be idiosyncratic among these groups of users. Underlying personality characteristics and mental conditions may influence the way a person responds to drugs, particularly at low doses. The best predictor of how a person will act while under the influence of drugs is how they have acted previously under the influence of the same drugs.