combat drugs

What are combat drugs made from? Are they made from plants? or artificial chemicals? Are they addictive?

Probably, like with modern drugs, there are varying answers to all of those questions. I'd also add that in addition to plants and artificial chemicals, it'd make sense for DE to make drugs from living and sentient creatures as well.

Boiled spleens of children!

Probably synthesised in some almost magical way using advanced recombinant technology and living organs for expression. EG boiled spleens of children!

Yeah, I honestly wish they'd given us some details about these drugs in the codex. How are they made? How are they administered? How do they work physiologically?

The thing about Dark Eldar is that, given their trend towards dilettante-ism, I doubt there's anything even remotely nearing any recognisable meaning of "standard". To me, outside of a few "designer creations" engineered by the Haemonculi, every batch of drugs is different, whether subtly or radically. We've got 6 different drugs listed, but to me those are just names for drugs that tend to have the same general effect.
In other words, no batch of Hyperdrenaline is going to be the same as another, because no Dark Eldar drugmaker is going to want to make the same thing over and over, and no true Dark Eldar is going to want to take the same drug over and over. They might still use different forms of adrenaline-stimulating/simulating drugs, but every batch is going to be cooked/brewed differently, consumed differently, made from different ingredients, or any combination of those.

One version of Hyperdrenaline might be entirely plant-based, with the oils extracted and purified, to be absorbed through the skin, while another batch of Hyperdrenaline might be created from the pineal glands of dozens of slaves, refined and condensed into a relatively small tablet.

I agree.

One haemy's concoction might make the body physically faster whereas anothers might increase perception of time - yet both do the same thing.

I do imagine there's a baseline standard for some of the stuff though. But pretty much any plant, creature or substance can be a drug!

"All things are poison, and nothing is without poison; only the dose permits something not to be poisonous.""

Even that though is, to me, just simplified for ease of game-use. Basically, there are so, so many different Dark Eldar drugs that "Hyperdrenaline" is more equivalent to "stimulant" than "cocaine", in a way. All stimulants have vaguely similar effects, but many have other effects, they come from many different sources, they work in different ways, and so on, compared to cocaine, which can be more or less pure, but not much else.

In other words, cocaine is cocaine, but hyperdrenaline is just the table-top term for one of countless drugs that have a roughly similar effect, but even that varies. One might stimulate the heart and muscles, allowing the Dark Eldar to move faster, while another stimulates the brain, allowing them to react quicker. Both make them "quicker", but in vastly different ways.

So, for example, we know there's a drug called Gravelotus. However, while there's a drug called Gravelotus, your other Wyches might not have actual gravelotus, but another drug that deadens the nerves while keeping them nimble and alert, while another has a drug that slightly boosts psychic potential, allowing them to "absorb" more of the pain drawn from slaves sacrificed before the battle. The closest they come to a baseline standard is a rough similarity in effects.