My expertise is more in the materials side of things than the pharmacology but I hold that some of the modern plastics, ceramics and composites are up there with drugs in top level complexity (see note in two paragraphs), indeed I have found myself going back to biology as I work my way into that world. Similarly I am a person/hang around with people who take great delight in making/figuring out how to make certain things from "ordinary" things and I have read the works of the Shulgins many times over.
Pills though: it should be noted that pills are not simple wrappers but often complex engineered delivery systems designed at limiting rate of delivery (why you are often told not to crunch high end painkillers) and point of delivery (certain casings are designed to break down and be absorbed in a given intestine).
Drugs: many will get hung up on the complex organic chemistry side of things and while there is no argument from me that many many things exist in that regime some are all too quick to forget that a lot of drugs are but simple inorganic substances (it is called lithium for a reason) or rudimentary organic substances (ethanol- as in booze is brutally simple to make).
Second question: it is implied a certain level of fitness for human consumption is desired but I will ask it outright; do you want this for human consumption (indeed next question would be long term or just will not kill someone outright but long term is a more risky proposition) or are you attempting a level of base purity for say reverse engineering (many high end drugs that need minuscule amounts to be effective will have masking compounds so separation becomes important).
Third question: are we talking "industrial" quantities (say the turning of "diet pills" into meth amphetamine for an area wide distribution) or again for the single use/reverse engineering level where quantity is not important. The general rule of thumb for industrial chemistry is very similar to the rule of errors in physics in that the more steps you have the less you process will yield.
Fourth question: are we talking my kitchen (well maybe not "my" kitchen) as is or here is £1K cash now go buy me a lab (indeed I could probably do it by the end of the day), similarly what is my budget for this (most of the home "cooks" that blow themselves up or make the stuff that kills people quickly are cheapskates and the rest tend to be those that rush it).
Also this common person is a vague term: A level (16-18 years old) chemistry contained sections on distillation (complex, fractional and simple), polymerisation, catalysis, acid and base reactions, suspension of substances in others, identification of compounds and on and on and on- all the basic lab techniques needed for this and every major bookshop in the country will sell a book on this meaning everyone who can read and transfer a bit of knowledge can play this game. Books for first and second year degree level chemistry, pharmacology and medicine are not restricted either. I have already mentioned the authors of one of the main works of illicit pharmacology. By similar token if you are holding a proper drug (rather than narcotic) in your hand it probably had the better part of a decade doing tests before you could get your hands on it; all that research is not that hard to get your hands on if it is not already widely published.
Fifth question: are we following a guide or are we playing the game for ourselves (I will dub this research grade). I hold that any fool that can cook something more complex than peas/oven food can follow a basic "experiment" guide (which is especially ironic that some of the chemists I have met can run a lab brilliantly yet they can not cook).
Dookieman argues that some of the chemicals are hard to get ahold of. Indeed while the "common person" (a trained type could probably mask their activities or talk their way out of it) probably could not walk into a chemical supply shop tomorrow with such an intent and leave without a facebag and free fast car ride in a black sedan:
Distilled (pure) water: I can make it or it is known as ironing water, they sell it for cars, they sell it for fish and on and on and on and on. I have a 50 litre tank in my hall right now. Essential for anything good.
Acids: high conc sulphuric I can get from car batteries still. The other "main" mineral acids are not that hard to get my hands on either.
Organic acids are a bit harder to lay my hands on beyond the basic ethanoic (what vinegar is made from) and the like but some fools persuaded the government that organic and water based paints and the like were the way forward so that is set to change.
Bases: your cleaning supplies are a good source.
Hairdressing holds many more (I am quite shocked at some of the stuff hairdressers can get and in what is no way intended as an insult they let the complete idiots do the hairdressing/beautician courses at most colleges* that I have looked at). *US bound college in the UK is something available for those aged 16-18 (16 is school leaving age for a little while longer).
Builders merchants: I was walking home the other day and some people down the road were redoing their driveway; the guy doing it had a whole bucket of off the shelf xylene sitting in his truck.
The stuff used to attempt to plug holes in car engines had some wonderful chemicals in when I was reading the list of ingredients a few months back.
It is nicer now but some of plastic bonding stuff in days past was useful here.
Kids chemistry kits: hold basic lab equipment and while modern ones are not quite as fun as say 80s/early 90s kits a bit of knowledge goes a long way. Tools to run a lab are not that hard to get or adapt from kitchen equipment.
Filtering: coffee filters anyone. Going back to cars for a minute all manner of organic solvents and inorganic solvents are there.
If adapting chemicals (useful for isolation and extraction) chemists/drug stores will have all manner of off the shelf and over the counter things that can be of use (citrates, sulphates, nitrates).
IR spectroscopy is a bit harder to find but more than a few chemical companies go bust/upgrade their stuff these days and if you only want a crude one it is not that far out for a competent engineer. While I would say this is a near fundamental for "research" if you are just following a guide you can probably get away without it. The kits to run "gels" are not hard to find either.
Going back to fish for a minute: the testing kits used in such things are amazing.
By similar token what once would have required a big machine probably now exists in a USB device on dealextreme.
Thus far I have not mentioned any of the more dubious methods either: dry cleaners use toluene/methyl benzene by the barrel (there are few better solvents), farms are almost always a source of interesting chemicals, schools often have all manner of chemicals in them (how many of you ever took home some magnesium; that is only the start of it), I have been in more than a few "abandoned" factories as well (the sort of thing where a chain is thrown round the front door and everyone is told not to come back Monday).
Dookieman also mentions boiling and melting point, for organics you also have denaturing point if you have enzymes in there.