A girl I went to school with told me she tried Ecstasy twice and didn't enjoy either time. I've heard of people having anxiety, puking, etc but is it even possible to not actually experience euphoria from this stuff? (I've only done it once and I couldn't imagine someone not liking it... it was like empathetic orgasim for 5 hours) but that was back in the 90s (haven't touched it since I got sober). She told me it was tested so I'm baffled by that...
Of course it's possible, there's no question about it. Bear Love has the meat of it.
I'll just add an example, but consider a person on mdma and then let's say...you get a call saying you got fired from your job. Your trip will turn the other direction and become massively dysphoric, probably much more so than if you weren't tripping. Basically anything upsetting, even just your thoughts, will wipe away any euphoria mdma (or any drug) might be giving you.
Or if you're simply not comfortable with altered states and you're not enjoying the effects, the same dysphoric quality will appear.
Obviously the effects from substances work in sync between the substance itself and the brain. If someone doesn't "like it," then they won't get the intended effects. So the mind can completely shut out and disregard the experience if they refuse it hard enough.
They're sort of like arguments. Even if you're right and the other person is wrong, the person that is wrong can still make some kind of 'counter argument' even if it's not addressing the main point, or not even accurate. Like an excuse, for example. There are so many points of view to cover for one point, that some of them would take a book's worth of writing just to address all of them beforehand.
In other words, there are so many possibilities with an individual's mind to be able to twist a drug's expected effects, that it's possible to get the exact opposite of what someone else experiences. This is why the sage phrase "drugs affect different people differently" holds true, and that's because the person can be very different, particularly their mind and individuality/bias (age, sex, race, beliefs, personal background and experience, genetics, etc). So like the example above, just like someone can refuse the truth, someone can refuse a drug's effects just by sheer defiance or unwillingness to accept. This doesn't even consider the outside forces that Bear Love mentioned that can also easily have a big impact.
The mind is super powerful, and it literally defines you, controls what you do and even what you experience. Substances are no exception, and it goes for all of them.