There are a number of things you may want to think about when addressing the issue of whether or not it is bad to drink water from plastic bottles. Bottled water certainly has a negative environmental impact, and it also poses some potential health risks. After weighing these issues, you may want to switch to glass or metal water containers, or start using heavy duty plastic water bottles which are designed to be safely used again and again.
From an environmental standpoint, it is very bad to drink water from plastic bottles. Plastics require a lot of energy for manufacture, and they do not break down very readily. People who drink water out of plastic bottles which are designed to be disposable are contributing heavily to the content of landfills, and to the general pollution caused by manufacturing the bottles, bottling the water, and shipping the water.
Since few standards govern the content of bottled water, many people are surprised to learn that their fancy bottled water is actually plain old tap water, retrieved from a reservoir. You can save a lot of money by drinking your tap water, which is also better for you, since tap water is regularly tested for contamination. This is also more environmentally friendly, as you can re-use water bottles by filling them at the tap.
However, when you re-use plastic bottles, you run the risk of encouraging bacteria to grow in your water bottle. Many bacteria love moist environments, and your well-used plastic water bottle can host a complex biofilm of organisms, some of which could make you sick. This is one reason metal and glass are preferable, because they can be easily sterilized, reducing the risk of ingesting bacteria.
There is also some debate over the answer to the question of whether or not drinking water from plastic bottles is safe, re-used or not. Some people believe that plastics leach chemicals which are potentially hazardous to human health, and that by drinking water from plastic bottles, people will ingest these chemicals, putting their health at risk. Other people believe that while plastics certainly do contain hazardous substances, the leaching ability of these substances is debatable. Despite several studies, no one has come down firmly on one side or the other.
A number of harmful chemicals are at issue in the debate, particularly the carcinogen di-2-ethylhexyl-adipate (DEHA) and bisphenol-A (BPA), a known endocrine disruptor. Studies do indicate that some plastics contain these substances, and there is a leaching potential, especially when hot water is involved, as the heat can encourage the plastic to offgas, and in the bottle, the offgassing vapors have nowhere to go other than directly into your water.
However, all plastics are not alike, and you should look at the recycling code on the bottom of a bottle for more information. Hard plastics with the numbers two, four, or five are generally viewed as safe to drink from and re-use, while bottles with numbers one, three, six, or seven may pose a leaching risk, and you should avoid them.