Did they really take heroin again?

I haven't seen it yet, but Renton doing drugs again isn't impossible, he is still very much the same man as he was 20 years ago, the only real difference being the drugs, so seeing these people again could definitely trigger that in Renton again

The shooting up scene is straight after Sick Boy gives Renton a hard time about giving Tommy his first shot of heroin and ultimately causing him to catch Aids through dirty needles. Then Renton returns the guilt trip to Sick Boy by bringing up the dead baby of his from the first film. Both then on a massive downer may believably have reverted to old habits to mask the pain.

Contrary to what other people here seem to think, I interpreted this scene as not actually occurring literally in the progression of the story. It seemed too surreal and dreamlike to take seriously, and had no consequential effect on subsequent scenes. Also, it seemed to out of character of Mark to be 'real' for me. I could be wrong though...

Because it's so moreish.

Tbh the idea of the scene being a vision of Spud didn't even occur to me.
I felt it was painful to watch (not in a bad way, I just really had hoped that at least Renton had overcome his addiction for good).

But the more I think about it as being a dream / vision, the more sense it makes to me.
Spud sees that Sickboy and Renton have a possibly destructive relationship and I think he is be afraid for them to go down the Heroin-route again. This would explain why he was sitting outside the room / not taking part and watching the two with a scared / sad look on his face. It would also explain why it was not mentioned in the movie afterwards.
Furthermore, like you said - it seemed out of character of Renton...

On the other hand they were really giving each other the guilt trip on that field.
Sickboy tells Renton that he`s responsible for Tommy's death and Renton lashes back at him recalling the death of his baby.
I guess this would be reason enough to go back on Heroin (hell, I probably would after this conversation).
Maybe they did take that shot after all....

Still, I prefer the idea of it being a vision. Would be just too depressing to think nothing / no-one had changed after 20 years...

I didn't consider that they might not really have taken it. It seemed wrong for Spud to be sat watching, clearly struggling with it.
For me, it was real, they took it. They are a bad influence on each other / Renton is easily led by Sick Boy.
But its there to be interpreted any which way...

To me, the point of the scene was to show the moment Spud had his lightbulb moment. Mark and Simon are simply reliving their youth by shooting up, after a downer conversation. Spud has been on the H since the first film. He wants to look forward, not back.

One of reasons why I assumed the scene really happened is "Chekhov's gun": we see Renton recovering drugs from his old bedroom, so this is him taking them.

--
"So I've got bullets, but no gun. That's quite Zen."