I would agree with those who say that there's little danger in (carefully) reusing these water bottles; but there are some reasons why there may be hazards associated with such use.

The temperatures required to mold the original bottles are high enough to completely sterilize them, and if they're kept clean until filled, the original filling should be pretty safe. Some tests (I believe Consumer's Union ran a few) have shown that the bacterial counts in most new bottled water samples are higher than the public water systems allow; but that's another story.

A newspaper straight from the press is almost perfectly sterile, due to the high temperatures used to dry the ink; but would you eat off an old one?

Most of the bottles I've seen can't be run through a dishwasher that's using water at safe "sanitizing" temperature, because the bottles will melt. If your dishwasher isn't getting to at least 140F, you're probably not getting any of your dishes "sanitary clean," but the bottles won't be much worse than the rest of your stuff - so your body is acclimated.

Plastics of the kind used for beverage/water bottles generally are at least slightly absorbent, and highly adsorbent. Contaminants in the original contents can embed themselves into the surface, and can cling tenaciously to the surface. Soap that you might use to wash these plastics tends to cling so tenaciously, due to the surface tension, that it is virtually impossible to rinse it off completely. As demonstrated by the numerous "algae blooms" due to runoff into streams and rivers, most soaps/detergents/surfactants, etc., are very good bacterial and algal nutrients; and if you can't get it off the inside of the bottle, you're feeding any bugs you put in with it.

Even chlorine bleach has a difficult time penetrating the adherent "contaminant" (soap and other "stuff") layer to a depth sufficient to completely "sterilize" (sanitize is a better term) the surface. Additionally, bleach has sufficient chemical activity to affect the bottle material in ways that might permit release of components of the plastic - such as the plasticisers already mentioned.

A fairly recent change in health laws here forced all commerical food services to get rid of their beautiful old wooden "chopping blocks," and replace them with plastic cutting surfaces. The theory was that the plastic boards would be run throught the (industrial strength) dishwasher after each use. This works, but subsequent tests have shown that any other method of cleaning the plastic surfaces generally leaves them with higher bacterial counts than were common on the old wooden blocks.

An industrial dishwasher should have a water temperature above 160F, and in some jurisdictions 170F is required. This will "sanitize" the plastic (but will melt your bottles). It will not necessarily remove the adherent "nutrient" contamination of the surface - but it kills the bugs.

Normal practice with the old blocks was to wipe them down with bleach after each use - and then to apply a vegetable oil (block oil) coating to keep the bleach from making the wood "nasty." Most reasonably fresh vegetable oils have sufficient "bacteriostatic" properties to prevent the rapid growth of crud on the block - at least for a while.

Without the oil, the cleanest surface you can get on the plastic boards by any "normal washing" leaves them as fertile breeding places for anything that happens to be there, especially if there's even a trace of moisture available.

The plastic boards are "safer," but only if you've got a 160-170F dishwasher and use it on them. You can't likely use such temperatures with your water bottles, because they'll turn into deformed lumps.

The plastic bottles are "safe" the first time, because they are "heat sterilized." You can't clean them that way, because they'll melt. There is no other method that reliably "sanitizes" flexible plastics to the same "level of safety" as the original bottles. Whether that level of "cleanness" is necessary, is something you have to decide for yourself. I'd say, in most cases, re-use is acceptable, if you fill with clean stuff, and watch for any signs of deterioration (taste, odor, color, etc.); but then I don't bother the dust bunnies much if they're not big enough to snarl when you pass them.

For general use of plastics for food, one should look at least for a "dishwasher safe" label - which means you can "sanitize them" in a properly heated dishwasher - or by pouring boiling water over them as the last rinse. In the absence of such a label, the only general rule is "the harder the plastic, the easier to clean."

John