In the short story Something more from Sword of Destiny why did Geralt drink the black gull to hallucinate in the first place? What was his goal?
At first I thought he drank it for helping heal his wounds, but then his dialog with Visenna (ostensibly his mother) suggests that this was not the reason:
"Later", she continued and pulled his shirt up to his neck, "a blood poisoning was added to it, like it's usually the case with bite wounds. It was slowed down. Of course a witcher elixier? That hepled much. But I don't understand why you took hallucinogens at the same time?"
So they didn't directly help with healing his wounds. It might be, as a comment points out, that he took them as painkillers. But I think there is even more to this idea and it's something else that Visenna says:
As part of this sympathy, a friendly advice, or if you want, an advice from the healer. Stop taking hallucinogens, Geralt. Hallucinogens don't help. Against nothing.
And this actually sounds much more reminiscent of drugs, even more so the motif of people (unsuccessfully) searching a solution to their problems in drugs. And I think this might very well be what Geralt ultimately did here.
Now what happened before his encoutner with Yurga and the fight on the bridge? Geralt learned that Ciri is likely dead, dieing when Nilfgaard attacked Cintra, and as a result of this he fled with Dandelion to the North (his third vision actually). And even more than that, he feels directly responsible for her death. We learn this in his visions.
The first of which is about a meeting with Yenefer during Belleteyn. He might even think by this point already that Yenefer died at the Battle of Sodden Hill (we learn later that this is not the case, but it's what he thinks when visiting the gravestone). And this vision ends with Yenefer reminding Geralt to go to Cintra and get Ciri, for Yenefer thinks she'll be in danger. (And she says this no less after talking with Geralt yet again about their inability to have their own children, reinforcing the idea of Ciri being Geralt's foster child.)
"Ride to Cintra. Ride there and don't deny this time. Don't do like back then...when you were there..."
"How do you know?"
"I know everything about you. Did you forget? Ride to Cintra, ride there as fast as possible. Bad times are coming, Geralt. Very bad. You got to make it..."
And in his second hallucination/flashback we learn how he was there the first time and denied to take Ciri with him. He said he doesn't believe in destiny and that he would take it upon him to tempt it and violate its rules.
But ultimately he realizes (or thinks) that it's not just on him to deal with the consequences of tempting destiny. Ciri is dead because he didn't take her with him. He did not make it there in time (it's his third vision where he learns about her death and where he's actually just on his way to Cintra). There's this telling line in his dialogue to Yurga:
"No, Sir. If I find something at home, it has to be predestined. And if you play with destiny, if you lie to it, it punishes you hard."
I know, the witcher thought. I know.
So while Geralt isn't always a man of big emotions, I think what we actually have here is a witcher struck by his guilt over a loss he himself feels responsible for.
So what does that have to do with him taking hallucinogens? Well, why do people flee themselves into hallucinations? He might either be trying to gain insights from it that make him see a bigger purpose in all that. Or he might just want to flee himself into his past experiences, bathing in self-pity over the mistakes he made back then, reliving those events yet again.
He is ultimately trying to search for answers in his past rather than looking into the future, which might as well be what Visenna alluded to when she said that "hallucinogens never help".
The excerpts here have been translated into English by me from Erik Simon's official German translation of Sword of Destiny.