Hello! So in the coming session the characters are going to descend into an underground laboratory, in which an awakened grade drug is being produced through an evil blend of blood-magic and technology.

I picture the laboratory to be similar to the underground caverns in the Ghost in the Shell live action movie, damp, dark caverns that lead into a room in which a number of awakened children are being hooked up to cybernetic harnesses that tap their spinal fluids through a system of tubes arranged in a ritualistic manner. Around the machine-park a number of blood-magic machine-cultists perform a continuous ritual and maintain the machines. I want the laboratory to have a strong background count, with the astral plane of a Dark-City-esque-vibe bleeding over. When walking down the corridors the players will hear weird sounds, and see odd things in their peripheral vision. Smells of iron will waft past, a taste of blood in their mouth. The two mages of the party will experience more intense visions, and hear whisperings of great power. The players will, on their way to the room with the children, get some vials of the drugs in their possession. The mages will feel a strong temptation to partake in the drug, and they will hear whispers from beyond about the power that will be bestowed upon them should they consume the drug.

After navigating the corridors they will reach the room mentioned above, and do some kind of battle with the cultists, and perhaps some weird spirits. The mages of the party will be all but useless because of the background count, unless they consume the drug, in which case they will be attuned (or aligned?), and more effective. The problem I have is that I don´t want to force them into taking the drug, since it will have some weird effects later on. So I don´t want the combat to be impossible without the drug. I´m also having this idea about the room they enter being so filled with background count that they embark on some sort of astral quest, either just the mages of the whole group.

I guess my problem is that I´m not sure how I should plan this rules wise, I could of course just free form the whole thing but I´m afraid that it will make the players feel like I´m railroading them.

Any ideas?

My party's mage has a magic 6, and a BGC of 2 has been enough that she will usually use cleansing, but is real hard on the adept. A 4 would mean they would both be pretty much unable to do their thing. If magic-based characters have non-magic skills to rely on, you could plan for using those to partially make up for magic not working. However, they probably haven't invested heavily into those.

Background count sounds a lot more interesting to GMs than it turns out to be in play. It also affects adepts more heavily (they lose powers and weapon foci), and adepts can't take cleansing metamagic. Also remember that opposing mages gain magic and force for every point of the BGC.

I did something similar in the past; if you feel mean you could impose Composure tests to avoid taking the drug.
What magic user does not pack a handgun? If they did not plan against BCG, it's their fault (however, it hits very much worse adepts).

The good old People Jars trope, eh? Maybe instead of vials, the awakened drug must be maintained in (and indeed delivered by) a living vessel such as a bee or else the drug "dies". The PCs could discover such bees and vacillate whether or not to take the drug, knowing that doing so would kill the bee. They could try taking the drug-bees away to sell / hoard for a rainy day, and find that bees don't live forever. That also allows you to limit how long the doses of drug the team gets their hands on remain a viable tool against future runs.

IIRC, world famous concentration camps have a BGC of 5, so unless this drug pit has been sucking the mojo out of victims in their millions it's unlikely to have a BGC over 3.

Although a BGC in an occasional encounter does tend to peeve Awakened characters (just like smoke grenades impair ranged combat, or medical-grade magnetic fields tend to get daikatana-dependent characters somewhat narked) that's all forgiven if the players see it has a purpose. I remember an infiltration run (playing a Night One bowman) with an artificially amped up background count some players were whining about -- until we broke into a section with captive vampires. Without the BGC, we wouldn't have been able to hurt them or keep them from regenerating, and a couple of Awakened characters operating at 100% would have been no use against a room full of vampires each operating at 100%. Once the team realised the BGC was helpful they suddenly became a lot more appreciative and cooperative. The mundanes were still 100% effective and the Awakened characters with grenades, stick-n-shock, etc. were perhaps 40% effective. But it was really good in the long-run too to remind the players that even when their silver-bullet default tactic isn't an option, they still have a lot of ways of ruining the antagonists' day if they're willing to engage.

So if you can allow the players some way of discovering, after some time, that the BGC is to their advantage (maybe it allows the technobloodmages to harvest an essential ingredient from something really nasty in the sub-basement... maybe a discarded failed prototype cyberzombie?) you can turn their perception of it into something positive.

So, you're in a grimy, damp underground laboratory where folks completely out of their minds brew something, but you don't even know precisely what it is. Oh, and the place does not just make every basement meth lab look like a clean room in comparison, it also screams "wrong" at the back of your head at full volume. Would your reaction be "oh, let me have a taste"?

I can think of two scenarios why anyone would even consider taking the drugs they found: The first would be if the alternative was certain death, which you don't want. The other would be that the PCs are not in their right mind, due to magical compulsion, drugs in the air, or something like that.

The second possibility (the PC's are not in their right mind) is also a bad idea, since the OP is concerned about railroading.

I would say that depends, if PCs have a realistic option to resist and clues to prepare themselves it's a challenge but not railroading.

While it's always good to provide PCs with options, it seems to me from the OP that indulging (or not) is a minor part of the evening's events.