Binding of a drug is typically a temporary action. Ligands: chemicals that bind and activate receptors. Efficacy: tendency to activate receptor; how effective will it be drug mechanisms. Drugs alter/affect the rate of biological functions. Pharmacokinetics: how drugs move thru the body. Tolerance: a decrease in the effectiveness of a drug that is administered repeatedly (need more to have same effect) Sensitization: an increase in the effectiveness of a drug that is administered repeatedly (need less to have same effect) Withdrawal symptoms: symptoms are opposite of those produced by a drug. Placebo: an inert substance given to an organism in lieu of a physiologically active drug routes of administration (depending on route, the effects will reach the site more slowly/rapidly) Up-regulation: adaptive change in receptors that occurs when blocked by a drug. Down-regulation: adaptive change in receptors with increased activation by a drug or transmitter. Receptors may become less sensitive drugs of abuse.