Why does lauging gas make you laugh?

Yes, it kind of does. Laughing gas is nitrous oxide, and it acts as an anaesthetic-type agent. It makes your brain feel a bit woozy in the same way that alcohol does. As a result, if you take some laughing gas, you fell a little bit drunk and a little bit cheerful. If you have enough of it, you start to feel a little bit sleepy, but it's very good at pain killing. If you're having an operation, it's sometimes used with other anaesthetics to kill pain and make you more comfortable. There is a sub-set of people in the population that have a particular form of a gene that is involved in making new blood cells. If they have this sub-set and have laughing gas, then it can affect their bone marrow in the long term. It can make your bone marrow work less well. Luckily, it's only temporary, but I don't think that I'll be inhaling lots of laughing gas.

Isn't it the same as nitrous oxide, N2O?

I believe so. But, to go back to your question, here are some of the things it can do.

-- Nitrous oxide oxidizes cobalt in vitamin B12  from Co1+ to Co2+, inactivating the vitamin and inhibiting methionine synthesis.

-- Nitrous oxide toxicity impairs nerve cell myelination.

-- Although vitamin B12 levels can be normal in nitrous oxide toxicity (since the assay may not distinguish between the normal and oxidized forms) hallmarks of laboratory diagnosis are elevated levels of homocysteine and methylmalonic acid.

-- Treatment of nitrous oxide-induced neurtoxicity involves administration of parenteral vitamin B12 and oral methionine.

-- Although most texts state that neurotoxicity from nitrous oxide occurs only after chronic use, this case seems to have come on after 2 weeks of high-dose abuse.
 
Signs and symptoms of nitrous oxide-induced neurotoxicity include: numbness and tingling of the distal extremities and impairment of vibration sensation, light touch and pinprick, proprioception, and gait.