The word that you are looking for is very long, which is why in drug slang (street talk) drug users shortened its name to four letters.
The four letters are MDMA, and it is short for the complete name of the drug which is Methylenedioxymethamphetamine. Another name used in the street is MDM, which is the same thing. This drug is commonly known as Ecstasy but they also have other names to hide what it is. One of the most common pseudonyms is created by switching around MDMA and making into "ADAM" as in "Let's go see Adam", or "Let's go get Adam tonight".
Other pseudonyms are "E", and M&M which is the short of the longer word. Parents are asked to check in urban dictionaries for the "fun" usage of drug names because that is a way to conceal drug use from parents.
ADDITION TO THE ANSWER:
Another drug that completes your M__A is Medusa. Medusa is an inhalant, which means that it is most commonly smoked. There is an FDA approved drug which is also called "Medusa" and its main characteristic is that its results are quicker and it remains longer in the body. Whether that correlation is true or not, others say that the name comes from the Greek goddess who would turn you into stone if you stare at her. Since "stoned" is jargon for being affected by drugs (particularly inhalants), there is a higher chance for this correlation than the latter. This drug is also called the "love fog" because it elicits uninhibited behaviors that lead to sexual and other illegal acts.