Methedrine is a "brand name" for an amphetamine that you can still buy, with a prescription from your physician. The later methamphetamines are more "popular" now, although legitimate medical use is much less common for either than was the case in the 40s and 50s.
Narcanon gives a brief history of the amphetamines and methamphetamines. The gist of it is:
[quote] First synthesized in 1887 Germany, amphetamine was for a long time, a drug in search of a disease. Nothing was done with the drug, from its discovery (synthesis) until the late 1920's, when it was seriously investigated as a cure or treatment against nearly everything from depression to decongestion. In the 1930's, amphetamine was marketed as Benzedrine in an over-the-counter inhaler to treat nasal congestion (for asthmatics, hay fever sufferers, and people with colds). A probable direct reaction to the Depression and Prohibition, the drug was used and abused by non-asthmatics looking for a buzz. By 1937 amphetamine was available by prescription in tablet form. Methamphetamine, more potent and easy to make, was discovered in Japan in 1919. The crystalline powder was soluble in water, making it a perfect candidate for injection. It is still legally produced in the U.S., sold under the trade name Desoxyn. During World War II, amphetamines were widely used to keep the fighting men going (during the Vietnam war, American soldiers used more amphetamines than the rest of the world did during WWII). In Japan, intravenous methamphetamine abuse reached epidemic proportions immediately after World War II, when supplies stored for military use became available to the public. In the United States in the 1950s, legally manufactured tablets of both dextroamphetamine (Dexedrine) and methamphetamine (Methedrine) became readily available and were used non medically by college students, truck drivers, and athletes, As use of amphetamines spread, so did their abuse. Amphetamines became a cure-all for such things as weight control and treating mild depression. This pattern changed drastically in the 1960s with the increased availability of injectable methamphetamine. The 1970 Controlled Substances Act severely restricted the legal production of injectable methamphetamine, causing its use to decrease greatly. [end quote]
There's quite a bit more on trends in usage, boiled down to - the vast majority of current use is illicit, and little commercial product is involved. Most of the street stuff is "home-brew" of variable, and often extremely dangerous, quality. It is extremely addictive, and a favored use is by pimps who, by fostering addiction can make their "crack whores" do just about anything - as long as they don't run out of the stuff. (But that's just street legend, of course.)
Illicit manufacture, usually by reduction of ephedrine (a common cold tablet) involves extremely toxic chemicals, many of which are commonly stolen from agricultural supply sources. Recent new laws in several states have made it almost as hard to buy ephedrine, epinephrine, or pseudoephrine hydrochloride (all common antihistamines) as to get a vistor's pass to the central vaults at Ft Knox.