Can you record over a DVD-R?

No. DVD-R and DVD+R are designed to be written to, once only.

The laser which is used to write the information onto the disc, causes a permanent change to the disc itself, referred to as 'Burning' for good reason. Once burnt it cannot be unburnt. or in other Words it cannot be recorded over.

To record over discs you need the special ones called DVD-RW. The RW refers to 'Read/Write'. The sensitive layer inside the disc, is designed to be altered by the laser and remains changed until the laser, with sufficient power, passes over it again.

The relative low power of the reading laser does not affect it.