Here's an anecdotal report of an adverse reaction that you can take for what it's worth. I've been meaning to write this up for a while. tl;dr vaporized MET fumarate may have caused lung irritation and bronchitis.
A friend and I were both pretty much completely recovered from what was either a bad cold or mild flu. Symptoms were largely gone, but our energy levels weren't where they normally were. We had been planning a psychedelic experience for the last two weekends, but had been sick/busy. On Sunday evening, we decided that the short-acting, lightweight, friendly, non-challenging experiences that many simple tryptamines like MET provide wouldn't put too much of a strain on us.
I have vaped MET fumarate (and several other tryptamine fumarates) several times before using a dual quartz coil vape pen to no ill effect, as has my friend. I'm aware that fumarate salts are unhealthy to vaporize, but I'm also aware that the actual amount of moles of maleic anhydride produced is quite small if not inconsequential. The one and only time my friend vaporized MET before, they mentioned to me that it felt bad in their lungs, a detail that I forgot about over the intervening months. I've come across a few other reports of vaporized MET feeling "caustic" or otherwise harsh when vaporized (would link to them, but that violates Bluelight rules.)
This time, I used a new vape pen with a bucket style heating element which my friend was unfamiliar with. After loading it up, they attempted to inhale before the powder had melted, and ended up inhaling and ingesting MET particulate. This engendered an instantaneous coughing fit, followed by the use of an inhaler. They again mentioned that it felt bad in their lungs, and that they would never vaporize it again. When I vaporized my portion, it felt more unpleasant than I remembered, but not especially noteworthy. I felt like this was probably due to the larger dose, caused by the marked increase in efficiency of the new vaporizer.
My experience started off familiar and pleasant, but quickly escalated into what felt similar to late stage sourness that I often get from mushrooms. Just a dark and oppressive, but nonthreatening heavy atmosphere--utterly unlike the warm positivity I had always experienced from MET before. This is possibly due to the increased dose, possibly due to some residual oxidized tryptamines in the bucket vaporizer, or maybe it just was what it was. I listened to some music and decided to go for a roll in the hay, which made for a strange experience (as the vibe was one of separation and alienation instead of warmth and connection.) The peak was over quickly and was quite manageable even though it was entirely unpleasant. My friend barely got above baseline, having inhaled rather than vaporized most of their dose.
The following day, my friend was very ill, far more ill than they had been the week prior. The first day was bad, the second day was worse--the fever, shaking, and chills made them consider going to the emergency room. Instead, they went to their GP and were prescribed strong antibiotics with a best guess diagnosis of bronchitis. The next day, they began to recover, eventually recovering ten days later. This was the sickest they have been in their life, apparently.
The next day, I found myself feeling like I needed to cough, my diaphragm squeezing little spasms of need, producing nothing to show for it. The same continued the next day, and the next day. Fourteen days later, I'm still feeling like I need to cough. I don't smoke or vape anything on a regular basis, so this is unusual.
Is any of this indicative of vaporizing MET fumarate as hazardous? Nope. Bronchitis follows a cold, and cold symptoms can persist for weeks after the infection is gone. Occam's razor would suggest that this is the more likely explanation. Still, the instantaneous sensitivity of my friend to the vapor both times and the dramatic turn of their health immediately afterward really stuck with me, and I'd be lying if I wasn't puzzled by my persistent cough. Alone, this still isn't significant, but before I ever with the other reports of adverse reactions to vaporizing MET on other forums, I thought I should make mention of it here just in case. For the life of me, I can't see why MET would be any worse than any other vaporizable tryptamine.
Needless to say, I won't be following through with my plans above to try it in a nebulizer - I'll stick to other tryptamines. It's a crying shame, too, because I quite liked vaporized MET and other ROAs haven't yet been worth the trouble or expense.