Some liver and kidney damage and the blood oxygen depletion is believed to be reversible. The damage to your body from inhalant use is dependant on the type of inhalant used, the length of time it has been used and other chemicals mixed in. Most inhalant users like to experiment with different types of products to see what gives them the best high. Absolutely, the potential of overdose is extremely high. The effects can hit so suddenly that it is too late.
Many substances are dangerous and deadly from the first use.
Nitrous Oxide is a good example. Most dentists will use Nitrous Oxide to relax a patient or use as a analgesic. Fatal if too much is inhaled. All of the government studies indicate that inhalant use begins in grade school and declines towards the end of high school. Very few adults use inhalants as a way to get high.