For a drug to work, it has to get to the place in the body where the problem lies, and that's why the science of pharmacokinetics is important. Enough of the drug has to stay at the site of action until the drug does its job, but not so much that it produces severe side effects or toxic reactions. Every doctor knows that selecting the right dose is a tricky balancing act. Many drugs get to their site of action through the bloodstream. How well the drug works generally depends on how well the targeted processes respond.
Drugs can alter the rate of existing biologic processes.
For example, some antiepileptic drugs reduce seizures by sending the brain an order to slow down production of certain brain chemicals. However, drugs can't restore systems already damaged beyond repair. This fundamental limitation of drug action underlies much of the current frustration in trying to treat tissue-destroying or degenerative diseases such as heart failure, arthritis, muscular dystrophy, multiple sclerosis, and Alzheimer's disease. Everyone responds to drugs differently. A large person generally needs more of a drug than a smaller person needs for the same effect.