There are two main categories of concern with regard to water bottle reuse:

As with any drinking or eating vessel made of any material, bacteria can build up inside a plastic water bottle if it is not carefully washed with soap and hot water between uses. If you simply refill a bottle without washing it, you run the risk of any bacteria left from the drinker's mouth or hands multiplying and building colonies on the bottle's walls and grooves. This problem is especially relevant for bottles that contained a sugary drink, as bacteria will use the sugar as a food source to fuel their multiplication.

The only characteristic that sets plastic bottles apart from other everyday dishes or cups with regard to bacteria is a small opening. Because the bottle's mouth is small, you typically cannot scrub the inside, an action which would help wash away bacterial buildup.

For safety: Clean your bottles regularly with hot, soapy water.

There has been a lot of discussion in the news in the past few years about the dangers of chemicals that could leech out of plastic bottles into the water they contain. This hysteria started with a student's senior thesis about an obscure type of plastic typically not used in water bottles. Her study suggested that the plasticizers could leech into water if the bottles were heated or frozen. Although the study was only concerned with a plastic not used in commercial water bottles, and its results were not subject to peer review by scientists, media outlets spun the story into a consumer panic.

The truth is less frightening. All materials that are intended for food storage or production are evaluated by the FDA for safety. This scrutiny applies to all plastic water bottles, just like any other food container. The FDA has determined that the plastics used in commercially available water bottles are safe for use with consumable fluids. All plastics emit small amounts of plasticizer over time, but the FDA has determined that these chemicals are harmless in small doses, and thus do not pose a risk to consumers.

The bottom line: Go ahead and reuse those water bottles. Just keep them clean.

Here are some websites with more information about plastic water bottle reuse:

http://en.wikipedia.org/wiki/Reuse_of_water_bottles

http://www.snopes.com/medical/toxins/petbottles.asp

http://www.plasticsinfo.org/s_plasticsinfo/sec_level2_faq.asp?CID=705&DID=2839

The reuse of plastic bottles is far from safe.

The story regarding leeching of chemicals from water bottles is not a student's hyesteria..the reuse of plastic bottles IS highly dangerous. The softer plastic bottles do break down and cause leeching and to call it just 'hysteria' is dangerous to all who read the answer to the question here and so reuse their own bottles.

Plastic bottles used for bottled water, especially the softer plastic bottles, are dangerous to our health and even more so if we wash them then reuse them! The plastic bottles contain toxic chemicals and in time the plastic begins to break down and releases these toxins and does so far quicker if the bottles are reused. If you can taste the plastic as you are drinking the water then you are drinking the plastic!

The bottles are even more dangerous if they are in a hot place... walking around with a bottle of water in your hand on a blazing hot day with the sun on it will make the toxins leach even more. If you are going to buy bottled water, then make sure it's in a "safe" plastic bottle.

To be certain you are using a safe plastic you can look for a symbol on the bottle, which is on ALL plastic bottles. The symbol is the recycling symbol. A number 2 HDPE means it is high density polyethylene, a number 4 LDPE - low density polyethylene and number 5 PP - polypropylene. If you have these numbers on your bottle, your bottle is safe. But, most plastic bottles in which water is usually sold are a number 1, and is only recommended for one time use.

Number 3 bottles contain polyvinyl chloride/PVC, which can leach hormone-disrupting chemicals into the liquids they are storing and will release synthetic carcinogens into the environment when incinerated. Number 6 bottles - polystyrene/PS, has been shown to leach styrene, a probable human carcinogen (a cancer causing substance), into food and drinks as well.

I'm sure you have noticed when picking up some bottles that they tend to 'flop' a bit in your hand as you grip them, these are the softest bottles and the most dangerous. If you are going to buy bottled water, always make sure it is a hard, rigid plastic.

I was reusing plastic bottles for years until I went to a seminar about mercury poisoning, held by the world leading anti-amalgamist, dentist and author, Dr. Hal Huggins. It was at the seminar that I learned the dangers of plastic water bottles. I certainly had my eyes opened that night, not only to the dangers of mercury in tooth fillings and the danger of plastic bottles but a whole host of other things...one of which is the mercury contained in recycled toilet paper!

You would think that it would be far safer to buy one of those hard plastic, colourful bottles we see in the shops...but NO..that's not safe either! These bottles, although have a safety number of 7, they leach Bisphenol A. Bisphenol A is a xenoestrogen, a known endocrine disrupter, which means that it disturbs the hormonal messaging in our bodies. Synthetic xenoestrogens are linked to breast cancer and uterine cancer in women, decreased testosterone levels in men and are devastating to babies and young children. BPA has also been linked to Type 2 Diabetes.

In 2006 Europe banned all products made for children under 3 years of age containing BPA. In 2007 there was a billion-dollar class action law suit against Gerber, Playtex, Eventflo, and Avent in Los Angeles for harm done to babies caused by drinking from baby bottles and cups containing BPA!

AMPS Society

The case against reusing plastic bottles makes many claims, but cites no evidence. In contrast, the case for safe bottle reuse has a wealth of peer-reviewed scientific evidence supporting it. A prime reference can be found in the following document from the International Life Sciences Institute:

ILSIPET.pdf

Note especially the following paragraph:

"There has been increasing public concern that certain man-made chemicals, if absorbed into the body, can act like the female hormone oestrogen disrupting the normal endocrine cycles and causing genetic disorders or adverse reproduction effects like reduced male sperm counts. It is important to stress that the chemistry of compounds that are used to manufacture PET shows no evidence of oestrogenic activity. There is a significant body of evidence that demonstrates that the use of PET is not a concern and is perfectly safe in this respect. PET and its components have no links with any reported endocrine disrupters [9,15,16]."

It is understandable that people would be scared about the transfer of plastic packaging materials to their food, but there is very little evidence to support this fear, and a large body of evidence to refute it.