This is so highly debated, and after looking on Reddit it appears some people take it all very seriously (which I think is probably a good thing moreso than bad). I feel I had my part in it as I've been posting the past decade about the importance of managing frequency and at one point even touted once a year..
Below I'll go over my evidence based opinion on the matter as well as squash some beliefs 99% of people have about frequency of use.
1-4 times a year is ideal.
5-7 times a year is safe with minimal problems.
8-10 times a year is pushing it and after a few years of use in this frequency range people typically start to feel the negatives (i.e. tolerance, comedowns, etc)
Anything beyond 10 times a year is too much and while some can use once a month each year on average, many cannot. Typically it takes a year or two at this usage to feel the nastiness but it'll come and once it does it's hard to go back.
Now above notice how I use the term "times per year" as an average. Most equate this to equally spaced intervals between each experience and god forbid you take two doses a week apart, you'll die from SS (sarcasm). Again I feel I've been partly to blame for this over the years as for most people this is probably the best way to go.
Reason being is many people have issues controlling their use. For people like these, telling them it's actually ok to dose close together so long as you meet your yearly average, is a ticket to problematic usage.
Which brings me to my point.. Dosing two days in a row is not dangerous, and in fact depending on how you supplement and care for yourself it could actually be safer.
It's called "preconditioning" and there are quite a few studies regarding mdma and this topic. Research shows that non-neurotoxic doses of mdma taken each day for four days before a neurotoxic dose attenuated the toxicity and serotonin depletion.
Take note the preconditioning doses were 10mg/kg once a day for four days, and the neurotoxic dose was 10mg/kg 4 times spaced 2 hours apart.
(Ref: Repeated exposure to mdma provides neuroprotection against subsequent mdma induced serotonin depletion in brain. 2009.)
This combined with the other neurotoxicity research out there indicates this to me. Dosing a single reasonable dose once a day for four days will be significantly less damaging than dosing those same four doses in the same night.
Anecdotal reports as well as my own experiences seems to indicate this is true. On to my experience...
I've been taking mdma for the past 13 years, I've been a good boy and just like hilo my dose has stayed the same all these years. The most I've taken in a year has been 8 doses.
With that said, almost every year 4 of my typically 6 or so experiences are back to back. I like to roll two days in a row at festivals. Typical wisdom should tell you I'm a vegetable by now, or that my second experience on those occasions is shit, neither is true.
It is my belief that this type of use could actually be safer for the neurons than spaced equally apart each year. There's only one catch that can make that untrue..
After an MDMA experience your antioxidant system has taken a huge hit and needs replenishing, if it doesn't get this you'll be vulnerable to the 400% increase in free radicals come the second experience the next night. Eating healthy and supplementing is key to make sure this doesn't happen.
Now while I believe in the safety dosing two days in a row (and by that I mean two separate experiences each night with reasonable doses, not a second dose the following morning to keep the party going) I don't feel it's smart to go beyond that. I've done it myself a few times with little repercussions but after two consecutive experiences the negatives outweigh the positives.. Plus if your keeping to a yearly average it's kind of a waste to have 3 days in a row and much easier to fall into problematic usage.
So there you have it folks.. It's not so much about closeness of your dosing as it is your yearly average.
-GC