Covalent bonds are very strong and in many cases not reversible under biologic conditions.

For instance, the covalent bond formed between the acetyl group of acetylsalicylic acid (aspirin) and cyclooxygenase (aspirin's receptor) is not readily broken. The platelet aggregation-blocking effect of aspirin lasts long after free acetylsalicylic acid has disappeared from the bloodstream (about 15 minutes) and is reversed only by the synthesis of new cyclooxygenase in new platelets, a process that takes several days.