Liver disorders often change the effect of drugs on the body—for example, by changing
How much of the drug is absorbed from the intestine
How quickly and completely the liver metabolizes a drug—for example, changing the drug into an active form or into an inactive form (a form that has no effect on the body)
How much of the drug is transported throughout the body
How quickly the drug is eliminated from the body
How sensitive the body is to a drug's effects
(See also Drugs and the Liver.)
How liver disorders affect a drug depends on the particular drug. Liver disorders can increase the effects of some drugs and decrease the effects of others. The drug's effect is increased if the liver is less able to inactivate a drug. The drug's effect is decreased if the liver is less able to change the drug to an active form or if the liver makes the body less able to absorb a drug or to transport it throughout the body.
A chronic liver disorder can make people more sensitive to the effects of certain drugs even when the disorder does not increase the amount of drug in the body. For example, if people with a liver disorder take even small doses of opioid pain relievers (such as morphine) or sedatives (such as lorazepam), mental function may deteriorate, and they may become confused, disoriented, and less alert. Mental function deteriorates, probably because the liver disorder makes the brain more sensitive to the effects of these drugs.
Because liver disorders are complicated, doctors often cannot predict how they will affect a particular drug. Thus, adjusting drug doses for people who have a liver disorder can be difficult.