Question

Can neurons be replaced?

Answer 1

In most cases, no. After the critical period of prenatal neurodevelopment, neurogenesis doesn't really occur. So if you're doing drugs and killing brain cells, they really won't come back!

Hippocampal neurogenesis is a small exception that's still debated over, albeit it cannot compensate for significant neuronal death.

The only neurons that are reliably "replaced" are olfactory receptor neurons. Basal cells differentiate into new olfactory receptor neurons about every two weeks, and form synapses with the mitral cells within the olfactory bulb, having converged onto the glomeruli.

This is precisely why neurodegenerative diseases are such an issue today. Most of them are like ticking time bombs, waiting to happen.