--"half life" in pharmaceutical terms is the approximate amount of time (because everyone's metabolism is different) for half of a given dose of a drug to be metabolized and "cleared" by the liver and/or kidneys (depending on the drug--gabapentin, for example, is processed by the kidneys and excreted basically whole, not as metabolites, and pretty much bypassed the liver).

It is more a measure of how long any active therapeutic effect can be thought to last, rather than an actual measure of how effective in symptom reduction a drug dose is.