So, my interest in these drugs is primarily from a therapeutic standpoint, so those without therapeutic effects are less within my wheelhouse. My ignorance caveated, I'll say with a grain of salt:
1. We have a shitty understanding of chronic MDMA toxicity in human beings. Rat and primate models of MDMA differ in the primarily damaged pathways (non-human primates it has a pronounced effect on serotonergic cells; in rats it has a pronounced effect on dopaminergic cells). It's also a "dirty" drug, in that it hits a number of different receptor types - which means its downstream effects are going to be in a bunch of different pathways. And even those receptors are dirty - the serotonin 2A receptor that MDMA hits is involved in a wide variety of cellular functions.
Beyond that, we do know that a number of MDMA metabolites are directly capable of forming free radicals (such as quinone and thioester compounds), and oxidative damage is a serious problem in any cell tissue, but especially in brain, where cell replacement is generally too slow to matter. In rats, we've directly observed the production of compounds due to free radical exposure after MDMA injection, strongly suggesting that that is a mechanism of damage in the brain. (We also find that in rats that over-express copper superoxide dismutase, an antioxidant mechanism, there's resistance to MDMA toxicity.)
MDMA has also been shown to induce neuronal apoptosis that is inhibited by serotonin2A blockage, but... serotonin2A has such a wide variety of effects, I won't even guess as to what the direct pathway of that interaction is, much less how to block it.
ALA is a fine anti-oxidant. However, I wouldn't put my eggs in that basket, for the following reasons:
1. Anti-oxidants mitigate the damage of oxidizers, the way that cops on the street mitigate street crime. Some of the cops are gonna do stuff you don't like (we also depend on free radical mechanisms for destroying nascent cancer cells and bacteria - ALA doesn't discriminate); some of the criminals are going to slip by anyway (oxidizing damage will continue to accrue, if slower).
2. ALA to my knowledge has been studied to the extent of "we gave rats ALA and MDMA for two weeks, and the ALA group seemed to have less grossly visible brain damage." I expect diminished brain function long before we have significantly detectable anatomic changes. (There's also a tiny crap study or two showing it didn't have any protective benefit at all, but I tend not to lean on 'tiny crap studies'.)
3. ALA as a protective mechanism relies on oxidation being the main mechanism of damage. It might be; we don't know. If the main mechanism of damage is serotonin-pathway-driven apoptosis, or serotonin-pathway-driven neuronal structural change, or dopaminergic-pathway-etc. then ALA will not play a meaningful role in preventing long-term damage. It would just be hitting entirely the wrong mechanism.
Bottom-lining it: if a patient said "I'm definitely going to do MDMA and you can't talk me out of it, should I take ALA to try and mitigate the harm?" I'd say yes, sure, it's not likely to do meaningful harm if you're not on it for years (or don't have a disease that causes you to have an impaired oxidative immune function.) If a patient said, "I'm curious about MDMA but only if I can do it safely, can I do it if I take it with ALA?" my answer would have to be, "There's no good evidence that ALA is protective, and several mechanisms of damage besides the one that ALA acts on. If you're not comfortable doing MDMA without ALA, I wouldn't do it with ALA."
As to how bad the effects are:
No one really knows. My professional opinion is that most of the low-grade effects are likely to be behavioral/psychiatric, and due to our generally poor ability to quantify and track psychiatric symptoms, and the social confounders that accompany drug use, we're unlikely to have any good idea about what their adverse effects are. My most honest answer is a profession of ignorance.
My personal opinion is that, assuming someone is healthy and doesn't have any particular underlying risk factor, almost none of these drugs are going to do meaningful neurological harm if done a couple of times. My bigger concern lies in (a) people with underlying psychiatric risk factors, (b) in combinations (alcohol + coke + etc.), and (c) non-neurological effects.
Opioids can absolutely kill you stone dead the first time you take them - but not from its neurotoxic effect. Given the spate of fentanyl being cut into the street drugs these days, and unregulated dosing, it's the one drug I'd tell people to stay away from like their life depends on it. Though obviously if you're taking a prescription pill one time at low dose, it's a different story.