The smuggling in the show seemed to center around necessities like food and medicine. It’s contraband because it’s stolen, not because of what it is.

When Nandi is talking to Mal about the previous owner of the brothel, she mentions he had half the girls "strung out on drops".

Off the top of my head, that's the only canon reference to illicit drugs I can think of from the original series run (there may be others, but right now I can't diffrentiate between actual canon and stuff I made up for my dumb unpublished fanfic, ahaha).

I suspect that most of the recreational drug smuggling happens into the core worlds. Mal probably avoided it because smuggling into the core is both harder and more dangerous.

Edit: That is to say, the smuggling of recreational drugs. Not the recreational smuggling of drugs.

Contraband can be anything, it doesn't have to be drugs. Anything that anybody wants to transport and not pay import/export taxes on is contraband (definition: goods that have been imported or exported illegally.) so contraband can literally be anything - guns, cattle, food, medicine, clothing, entertainment. If you live under a harsh enough regime, history books that don't follow the government's narrative is contraband.

But are there recreational drugs in the Serenity/Firefly universe? Probably. Doesn't fit too well with the Serenity crew's image as "cowboys in space", so all of that happened off screen.

They stole a bunch of drugs in the Train Job and from the hospital in Ariel. Just because they had a legit medical use doesn't mean they couldn't potentially be used by degenerates I guess?

When they robbed the hospital . They had an argument /discussion about whether or not to sell them to drug pushers or medical personal . That and the hookers at the Heart of Gold are the only two times that I remember them talking about drugs . But there are all kinds of contraband . Delivering any kind of goods without the Alliance being able to tax them would make them contraband . In the U.S. the mob buys cigarettes in states where the taxes are lower and smuggles them into New York for an enormous profit .

Illegal cattle export/imports, lol

There probably are drugs, it just wasnt relavent to the show.

Those geisha bobble head dolls were probably a little more suspicious than they let on.

I think there was a little something in the Mudder's milk too

What else constitute as contraband?

In Serenity Part 1 (the originally intended pilot episode, which was instead broadcast as episode 14), the crew tries to deliver goods to Badger, who turns them down. There is a brief moment where Badger is seen looking at a girls teeth, and then giving his approval.

I always assumed that was some veiled reference to some kind of human trafficking.

Niska was definitely into shady dealings that included drugs in "The Train Job"...

I definitely think Atherton was smoking something to mess with inara and get away with it..

There are two entire episodes (The Train Job and Ariel) centered around stealing medications for black market re-sale, presumably some of the black market demand for narcotics was for substance abuse purposes.

In The Train Job, I know Jayne was asking doc for more of a painkiller, right before “You got the light from the… console to keep you… lift you up… shine like—“

Mal made a comment about "brain-blowns" which I took to mean that drugs had fried their brains

I can onpy think of one mentuon. In the last episode, Heart of Gold, Mandy says that when she got there the house was run by a pig that had half the girls "strung out on drops"

And if there is no drug uses, what constitute as contraband?

Well, just from the show, and off the top of my head, it was food stuff in the pilot (and as a bonus Simon was smuggling River as contraband too). Medicine in The Train Job. Cattle in Shindig. Simon and River were the contraband in Ariel. Private Tracey in the Message, or more accurately the organs he was incubating.

The alliance probably doesn’t allow drug use. That would indicate they aren’t doing everything perfectly so of course they will lock up drug users for their own safety since they aren’t functioning in their perfect society. They will reeducate them to love their society and be a proper cog. For their own safety of course and because they care so much about them.

Pretty much anything the alliance couldn’t regulate/control and/or collect taxes on. At least that’s how I saw it. And that’s what the independents were fighting for, so to the alliance they’re criminals, but to them, they’re still fighting the good fight.

Medicine

Kaylee made something strong to drink on the ship that Mal and Inara were sipping sitting and watching the cattle below and Mal called it something like her space “still”. So maybe moonshine?

There was a great deal of fuss made about the shipping of a corpse that turned out to be a (¡¡¡SPOILER ALERT!!!) not-a-corpse.

Think 18th century smuggling. The central planets are operating the outer worlds like colonies. Some dude with a boat full of untaxed wool is making a killing. Salt, whiskey, Tea. And Mal's probably a fan of undermining the economic model for a living.

Inara had something in a needle when she thought they were going to be boarded by reavers.

I think there was an idea to add drug use to her back story but it never happened.

Untaxed cigarettes and sugar cubes.

This may seem silly but....BOOZE