Drugs bind their targets or receptors using chemical forces, such as covalent bonds, electrostatic interactions, and hydrophobic effects. Covalent bonds are strong and irreversible and rarely occur except in toxic situations. DNA-alkylating anticancer agents form covalent bonds with cancer cells' DNA and arrest their cell division. However, such agents are nonselective and modify the surrounding healthy cells' DNA. In contrast, non-covalent interactions are weaker but more selective because a drug must precisely fit its target to form such bonds. Most drugs and receptors interact via electrostatic interactions. Electrostatic interactions are strong between oppositely charged ionized molecules and weak in the case of hydrogen bonds and van der Waals forces. Another type of weak interaction occurs due to the hydrophobic effect when drugs interact with the internal hydrophobic pockets of receptors and exclude surrounding water molecules. Such selective interactions of drugs with their receptors modify the activity of target cells without affecting other cells.