Drugs mainly interact with the receptors by means of chemical forces or bonds. There are three major types of drug receptor bonds: - Covalent - Electrostatic - Hydrophobic Covalent bonds are very strong bonds and in most of the cases they are irreversible under biologic conditions. For example, the covalent bond between the acetyl group of aspirin and cyclo-oxygenase enzyme (target enzyme present on the platelets) does not breaks easily. The platelet aggregation effect of aspirin lasts long after free acetyl-salicylic acid has disappeared from the blood (about 15 minutes) and it is reversed only by the synthesis of new cyclo-oxygenase enzyme in new platelets which takes a long time. Hence the effect of aspirin is seen after the drug is stopped. Among the drug receptor interactions, electrostatic bond is much more commonly found than covalent bond. The electrostatic bonds vary from relatively strong linkages between permanently charged ionic molecules to weaker hydrogen bonds and very weak induced dipole interactions such as van der Waals force. The electrostatic bonds are weaker than covalent bonds. Hydrophobic bonds are usually very weak bonds and probably important in the interactions of highly lipid soluble drugs with the lipids of cell membranes and perhaps in the interactions of the drugs with the internal walls of receptor “pockets”.