Medications can do many things. But certain effects aren't exactly what either you or your doctor had in mind.

Failure: Antiretroviral drugs quit working. (Note: It's the drugs that fail, not the people.) The evidence for failure -- a rising viral load, often combined with falling CD4s. But it's a matter of degree. How high an increase? How fast? How consistent? If the viral load climbs from undetectable to 100,000 and the CD4s are in free-fall from one test to another, give it an F and move on. But a less dramatic viral increase, or one that falls and rises from test to test, might just reflect test variability.

Side effect: The drug has an action or effect other than the one intended. Side effects can range from headache to foot numbness, fatigue to hyperactivity, depression to euphoria, sleepiness to insomnia. Depending on the severity of bad side effects, dose reduction or switching to another drug may be required. Luckily, some side effects are good. For example, in studies of a growth hormone (Serostim) for reversing wasting, the drug boosted white blood cell counts, an unexpected bonus.

Toxicity: Defines how poisonous a substance is or how it interferes with bodily functions. The toxicities vary. Examples: Bone marrow suppression that can cause loss of red blood cells (anemia) or white blood cells (neutropenia), and liver or kidney damage. The drug dose can greatly influence how toxic a drug is. And some people are simply more sensitive, suffering toxicity at even moderate doses.

Allergy: The body reacts to a drug with an inappropriate immune response. The resulting symptoms range from mild -- the common no-big-deal skin rash -- to life-threatening, such as the very-big-deal severe rash that's part of the Stevens-Johnson syndrome experienced by some people beginning nevirapine (Viramune). Although rare, this syndrome can cause serious lung, heart, kidney and intestinal problems. And it's tricky because the onset may be nothing more than flu-like symptoms, low-grade fever and minor rash. So no matter how mild a symptom is, always report it to your physician. Stopping a drug in time could save your life.