Liver cells, called hepatocytes, play a vital role in the functioning of the liver. If these cells begin working too hard to remove chemicals from the blood, or if they are harmed by other infections (e.g., hepatitis C virus), abnormal chemical reactions can occur that can damage these cells. There are actually three ways in which this can happen:

• Taking a very high dose of a drug. If you were to swallow a high dose of an anti-HIV drug or another medication (i.e., taking many pills when you are supposed to take one or two), this can cause immediate and sometimes severe damage to liver cells. Almost any drug, if an overdose is taken, can cause this type of liver damage.
   
• Taking standard doses of medication for a long period of time. If you take medications on a regular basis for a long period of time, there is also a risk of damage to these liver cells. This usually occurs after several months or years of taking certain medications. Protease inhibitors have the ability to cause damage to liver cells if they are used for long periods of time.
   
• An allergic reaction. When we hear the term "allergic reaction," we often think of itchy skin or runny eyes. However, allergic reactions can also take place in the liver. If you are allergic to a particular drug, your immune system can cause your liver to become inflamed as a result of interactions between key liver proteins and the drug. If the drug is not stopped, the inflammation can worsen and can cause serious damage to the liver. Two anti-HIV drugs known to cause such allergic reactions (sometimes referred to as "hypersensitivity") in HIV-positive people are Ziagen® (abacavir) and Viramune® (nevirapine). Allergic reactions such as these usually occur within a few weeks or months after the drug is started and either may or may not be accompanied by other allergy-related symptoms (e.g., fever or a rash).