A reddit user asked me the other day if I had had any experiences with bad acid, and how to make sure you were buying good acid.
I think most people would define bad acid as some form of LSD that is guaranteed to cause you a bad trip. This is something that a lot of people are anxious about, because no one wants to doom themselves to a horrible experience like that. But if it's defined like that, then "bad acid" does not exist. Pure LSD is only one chemical, and no forms of it are "spoiled" or wrong.
Bad trips are caused by only a few things:
bad setting and environment
a dose that is higher than one is comfortable with
unexpected difficulties, whether external or internal
NOT "bad acid"
However, just because you are getting "acid" doesn't mean you're getting pure LSD, and this is where the problems occur. Most of the time, your biggest worry is going to be whether or not someone is going to sell you blank paper. This really sucks, but obviously poses no real risk besides disappointment. More rarely, people can and will cut it with other chemicals. (In my opinion, this is more of a danger with liquid LSD because not all chemicals will stick to blotter. If you just bought a pill, it's probably not LSD.) If you are expecting an LSD trip and you're not prepared for what those unknown chemicals do to your psyche, your likelihood of having a bad trip is a lot higher (see bullet point number four). Even if what's in your acid catches you off guard, you can still avoid having a bad trip by staying calm and taking the precaution of not tripping alone. For more information on bad trips, you can take a look at my post How To Avoid a Bad Trip.
So what can you do to help ensure that your acid is legit? I will give you the same advice that I gave the reddit user: always buy from someone you trust. It's cynical, but it's something that must be taken into account for as long as drugs users are forced to engage in the unregulated black market: someone who knows you personally would have to take responsibility for selling you a bad product, and therefore they will usually refrain.