While some research has shown that certain chemicals can leach into your water over time if you reuse disposable plastic water bottles, there's no indication that these are hazardous to your health.
However, cracks in the bottle from wear and tear are the perfect spots for bacteria to hide. Disposable water bottles also won't last forever and will end up in a landfill, so you're better off investing in a reusable water bottle that can be recycled down the road. If you do decide to reuse a disposable plastic water bottle, make sure that you wash it with warm water and washing-up liquid before every refill.