Having hired LOTS of chemically dependent employees since 1982, here is my take on the different ways the drugs affect you. Of course I am referring to hard core abuse, not the occasional sample.
Alcohol: makes you unreliable to get to work, you work slower, and over the years it makes you stupid. Decent folks otherwise, although the drunks tend to be more emotional people with ego problems. When it is real bad, their desire to get back to the bottle will keep them from working late on a project or it will make them work fast and sloppy just to be done. Quitting time is project number one!
Pot: Lazy, don't care attitudes. Not real good attendance, 'cause they just don't care. The THC never quits coursing through their veins, and they don't seem to realize that they are affected all the time, not just when they are stoned. Usually reasonably easy going, they are also very abusive to equipment, because they JUST DON'T CARE. They are the VERY worst "bring your drug" to work people, they are also the hardest people to manage because they won't do what you tell them to do. Socially engaging, most of them try hard to be popular with co-workers, while at the same time letting you (the employer) know that they are socially rebellious. They are the most divisive, "us-against-them" employees you will hire.
Meth, in any number of permutations: These are the fireballs on the job! They will work fast and furious some of the time, and other times they are very down, both in production and temperament. They tend to have real social problems, and will disappear without answering the phone for days at a time. Getting quality work out of them just isn't going to happen, because they are either cranked up, tearing up equipment or property, or they are "down" and can't hardly bring themselves to do a good job. They also seem to be more inclined to having problems getting along with other folks at work. These folks will tell you anything, they are the worst liars going.
Cocaine, in any number of permutations: generally, these are the hardest people to detect on the job, unless they are really stretched out. When they are going on binges, they disappear like the meth heads. Generally, you can find them easiest because they are the guys that are always out of money on Monday morning. They tend to be thin, underfed, and generally less productive on the job because they don't eat right. (also a BIG problem with the meth-heads) The cocaine users really tend to binge the paycheck on the cocaine, and balance out the rest of their time getting drunk. These folks are usually pretty likeable, especially since they are always being extra polite so that you will loan them money, share cigarettes with them (I don't smoke, but I see it), or otherwise fill some need of theirs. They tend to be the more helpless guys on the job.
Heroin and other really bad stuff: these folks are generally so spooky and sunken eyed looking that you don't keep them around long enough to find out what they are really like. They are consumed by their multiple drug dependencies, and they look like it. I don't think I have had too many of them working for me.