many drugs you can keep taking even pretty heavily before anything bad starts to happen. If you split drugs up into different categories it would be something like this:
Really Soft Drugs -> LSD, Mescaline, Mushrooms
Soft Drugs -> Marijuana, 2c-b
Firm Drugs -> Ketamine, Nitrous, MDMA
Hard Drugs -> Meth, Heroin, Cocaine
Really soft drugs = use away you are never going to have significant problems from these unless you have like a family history of mental illness or something, or a bad trip triggers some mental problems, which happens really rarely and possibly not at all if you are smart about how you use them.
Soft Drugs = Use away but they can become kind of habit forming in a significant number of people, could impair your ability to function a bit if you use them frequently (whereas really soft drugs you are very unlikely to use them that frequently), could have problems from it depending on how you use it, but typically really safe and you can go hard with it
firm drugs = Not going to cause problems if you use them in an intelligent fashion, some people get very carried away and probably get permanent brain damage to various extents from doing so, can cause organ damage and other health problems as well, require more work to use safely, can be very habit forming but not as powerfully addictive as hard drugs (more of an immediate desire to redose than a typical long term addiction, in the drug scene we call drugs like this "moreish" rather than "addictive"), probably most people keep their use in moderation, the risk of health problems increases significantly depending on how much you use, starting from pretty low and ending up at pretty high, can kill you but pretty unlikely to do so even with true abuse
Hard Drugs = Cause problems in a significant set of the population that uses them, but not all users have serious problems, highly addictive to some people and addictive to most people but some can keep their use under control, kind of playing with fire to try these drugs, I managed to and I am fine but have seen them all ruin various people, inherently negative to your health both to a moderate degree in the short term and really very much so in the long term in many cases, the most likely to kill you or cause you to turn into a scumfuck criminal, etc.
That's just a rough classification system anyway, more research needs to be done on individual drugs before we can properly classify them, but my point is that MDMA is often seen as a really soft drug, but in reality it is trending more toward a hard drug in a lot of ways.