Every single reply on that thread pointing out the imperceptibility of marijuana has been downvoted to the negatives. The top voted reply is quoting a person who clearly has absolutely no knowledge of science. After stating criteria for the supposed permissibility of marijuana, he advises the reader to try homeopathic medicine.
Anyway, the three conditions he gives are never met.
1) The American Medical Association vehemently rejects the use of marijuana as medicine. It has reaffirmed this position and explicitly stated that it opposes the legalization and use of marijuana. The American Psychiatric Association states "There is no current scientific evidence that marijuana is in any way beneficial for the treatment of any psychiatric disorder. In contrast, current evidence supports, at minimum, a strong association of cannabis use with the onset of psychiatric disorders.
2) Marijuana is not approved by the FDA so there is always an alternative medicine. The people thumping about the amazing effects of marijuana have no clue how medicine works. If marijuana has any chemicals with medicinal effects, that substance will be isolated, studied, and then synthesized in a laboratory in order to make a drug. That is the potential for marijuana as medicine. You purify the substance which has the desired effect. You don't prescribe a medicine that has God knows how many other chemicals which can cause drug-drug interactions. You don't see physicians telling their heart failure patients to chew on digitalis; they prescribe digoxin to them.
3) It is used only in accordance with need and no more. There's no point in even arguing this point since the above two are enough. But let's be realistic. The people getting medical marijuana are getting it in order to get high. Type in medical marijuana on Google and you'll find countless websites telling you what you need to say to get a card. There are unscrupulous doctors who literally market themselves as 420 docs. You come in, pay the fee, you walk out with the prescription. No one is being fooled. It's being used solely to get high.