I'm just going to list the most harmful, and least harmful in no particular order.
Harmful
1. Chloroform, carbogen and other inhalants - not nitrous or ether, but stuff like gasoline, jenkem, computer duster or glue. Most of them are carcinogenic or extremely toxic. You're depriving yourself of oxygen too, which adds even more to the brain cell casualties.
2. Unrefined fungus piss. Stuff that comes in plastic bottles or unfiltered homemade brews still with dead yeast on the bottom. Dark liquors like whiskey and tequila are the worst, because theirs more toxic acetylaldehyde being made (based on studies). Definitely snaps off a few dendrites.
3. Cheap processed tobacco. Those cheap cigarettes that are based on shredded tobacco waste and reconstituted tobacco sprayed with nicotine, and have like 4000 carcinogen, radioactive, or otherwise toxic chemicals added. Unfiltered ones especially.
4. MPTP - industrial toxin that used to be passed off as an analogue of meperidine (demerol) in the 80's. not really around anymore, but think it's worth a mention. It's the biggest reason the federal analogue act got passed. So it proved much more harmful to us druggies than just causing parkinsons.
5. PMA - This stuff is mostly passed of as ecstacy. Very dangerous stuff that usually leads to convulsions, coma, or death. Also dangerous increase in BP, temp (fever), and pulse. labored breathing, nausea/vomiting, muscle spasms, etc. and with little to no appreciable effects.
Safe. All in moderation of course, not done regularly.
1. N,N-DMT, 5-MeO-DMT, and generally speaking all tryptamine psychedelics.
2. GHB (medical grade).
3. High potency medical grade cannabis used orally or vaporized.
4. Nitrous oxide. Until a certain point, it's shown to be neuro-protective. If it's only done occasionally than vitamin B-12 depletion can be avoided, unless you want to supplement with it.
5. Nootropics like racetams, DMAE, gingko biloba
6. Body building drugs like HGH, DHEA, and some anabolic steroids.
7. Morphine
Theirs many others, but I'm not going to list all of them or explain why unless asked.