Fluorine forms a stronger bond with carbon than the carbon-hydrogen bond it replaces. This increases the energy required to metabolise the drug molecule, reducing the rate of the cytochrome P450 isozymes.
This increases the bioavailability of the drug by slowing its metabolism. Therefore, the drug is present for more time, able to have a greater biological effect.
Having a longer half-life can be beneficial – by reducing the frequency and number of doses, but it can also cause problems. If a drug is present for too long, it may begin to have off-target or toxic effects. Therefore, a balance must be found for the optimal half-life.