The last several months numerous articles have been written on how the US public is consuming trace amounts of pharmaceuticals such as antibiotics, hormones, mood-altering drugs and pain killers in each glass of water they drink. The pharmaceuticals wind up in the water supplies in 1 of 3 ways: they may be excreted unmetabolized by humans or other animals consuming them, topically applied drugs may be washed off, or unwanted drugs may be flushed down the toilet. Some pharmaceuticals are not degraded during waste water treatment and are released back into waterways in concentrations in the range of parts per billion. From there they find their way into our drinking water.
Studies have show that fish are effected adversely from female hormones in concentrations as low as 5 to 6 parts per trillion. As far as humans go, the fetus and newborn are going to be most susceptible the pharmaceuticals; but to date there has not been enough studies done to determine the potential effects of these contaminants on human health.
Your safest bet if drinking tap water is to use a reverse osmosis unit. This technique will filter out unwanted pharmaceuticals.