Thanks to its use in medicine, Ketamine is relatively safe compared with most recreational drugs. There have been numerous human clinical trials and its effects, long and short term, are well documented and understood.

Nevertheless, recreational drug use is not controlled medical use

overdosing
Overdosing on a self-administered dose of Ketamine is nearly impossible because it has a wide safety margin and you will pass out well before it can kill you.

However, Ketamine is not a good drug to take outside the home. You're quite likely to fall down, get run over, or at least make a complete fool of yourself.

Talking, moving or even going to the toilet is not easy; it's important to be in a safe environment with, ideally, one non-participating friend.

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physical dangers
At high doses, Ketamine can be physically incapacitating, even paralysing. Make sure you extinguish all cigarettes, candles and anything flammable that could be knocked over.

long term dangers
Some emerging research suggests that heavy and prolonged Ketamine use can cause brain damage, in the form of 'Olney's lesions' or 'vacuoles.'

However these vacuoles were found on rats injected with Ketamine and experiments on monkeys have failed to produce similar results. This is probably one reason why the Federal Drugs Administration (FDA) in the US has not removed medicinal Ketamine from the marketplace. (1)

1. Ketamine: Dreams & Realities p.241