Heroin also improves depression within minutes. Ketamine is even more dangerous, and may cause severe cognitive difficulties with long-term use like other NMDA antagonists like ethanol and the benzodiazapenes (which, it should be noted, actually worsen depression in the long run).
You are touching on some legitimate reasons for skepticism, however I think it's a bit more complicated than you are suggesting. Clinically, the response to alcohol, benzos, and heroin on mood is reported less as improvement than making the symptoms easier to ignore. I'd be surprised to see those agents prompt this kind of change on the MADRS. Plus alcohol and benzos act primarily through GABA, and heroin via mu opioid receptors, so the mechanisms are not totally comparable.
The long-term effect of ketamine is not yet understood, but establishing safety and effectiveness in these very small trials is the first step.
ketamine is being studied as single (for depression) or occasional (two week interval in the case of this study) dose, at which frequency there are no known deleterious effects. if you have evidence to the contrary, let's see it.
- The full methodology isn't actually published anywhere.
- The rankings are creating by combining a lot of different factors that don't have anything to do with each other, e.g. by combining harm to the user with harm to society. This means that drugs like coffee end up being more dangerous than drugs like heroin, simply because more people use coffee than heroin so the total social costs are greater.
- The harms for drugs are measured as they are typically used, rather than correcting for things like differences in demographic and route of administration. This leads to drugs like heroin looking more dangerous than they are, because people who have drug abuse problems tend to gravitate toward drugs like heroin. (Whereas people who use, say, Khat tend not to be the worst of the worst as drug abusers go.)
- The harms of the drugs caused by prohibition are not accounted for. (E.g. they are counting people using dirty needles and impure/unknown/fake drugs as being a harm that stems from heroin, but they aren't counting using dirty needles and fake Starbucks as being a harm that stems from drinking coffee.)
- They're not accounting for the benefits of drug use, only the harms.
I could keep going, but I think I've sufficiently proven my point and then some.
Maybe less dangerous in the immediate term from overdose, but isn't ketamine potentially rather dangerous to the brain? Wikipedia seems to suggest that frequent ketamine usage causes memory issues, and there might be possible brain damage.
Whereas opiates, to my knowledge, don't actually have any serious side effects, or long term health issues (apart from constipation and perhaps lowered testosterone). Apart from the overdose risk, that is.
Ketamine blocks the NMDA receptor, which is thought to be the receptor that causes memory formation. So it is expected to affect memory.
The brain damage effect is uncharacterized in humans. Several NMDA drugs cause this effect in rats, but none of them have been found to cause devastating problems in humans except PCP.
There is some suggestion that opioids cause cancer, but the only study was small and observational.
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