A drug's half-life is determined by finding out how long it takes a medication or drug to be eliminated from blood plasma by one half of its strength. The drug's half-life is usually measured by looking at how medications degrade when they are administered via injection or intravenously in an IV drip. Theoretically, one dose of a drug administered by injection or through an intravenous (IV) line could be measured by looking at blood levels to see when the drug has reached half strength.

There are some medications that have notoriously short half-lives, which means the body eliminates them and reduces them in strength very quickly. These include many over the counter pain relievers and medications that are used as tranquilizers like benzodiazepines. In fact, sometimes a short half-life can be a problem when you take things like Valium® or Xanax®. These may temporarily promote a sense of calm, but that feeling can wear off very quickly. This can create the need to take more of a drug than is really recommended.

Other times a drug's half life is fairly long, which means that it may stay in the body for a very long time without significantly diminishing in strength. Certain blood thinners like warfarin (Coumadin®) have long half-lives. This can have medical significance if a person needs to have any type of surgery. Because warfarin thins the blood and reduces the blood's ability to clot, people taking this medication may need to stop taking it several days before a surgery to reduce risk of excessive bleeding during a surgery.

There is a specific formula to determine a drug's half-life, expressed as t(1/2) or time it takes to reduce strength by one half. Two important things that scientists look at to determine a drug’s half-life are volume of distribution and clearance. Clearance can be defined as the body’s removal of the medication from blood plasma. Volume of distribution refers to the way the drug gets distributed to other parts of the body.

Another important term when considering a drug's half-life is bioavailability. This is the amount of a medication (or any substance) that is available for use in the body. When we take oral medications they must pass through our stomachs first, so they are less strong when they reach our circulatory system, and a short half life may mean we use very little of a drug if it is already near half strength when it gets to our bloodstream. Understanding the bioavailability of oral medications and a drug's half-life can help pharmacists and doctors determine appropriate dosage of medication.