There isn't really any evidence for drug use in the Viking Age with the obvious exception of alcohol. You may have heard on the internet that berserkers used mushrooms to get into a state of frenzy and fearlessness before battles. This theory crops up from time to time. However, it is not really based in reality. As best as I can reconstruct it was proposed in analogy to circumpolar shamanistic practices, which could involve fly agaric in order to induce the dream flight so typical of shamanism. However, there is no evidence that the Vikings used fly agaric or any other kind of hallucinogenic drug. There are two exceptions to this, I'll come back to them later.

Alcohol on the other hand did play an important role in Viking Age sciety, as it did in the societies before and after it. We know of this not only from the written sources (which are usually later) and from mythology (also reconstructed from later sources). Just think of the poetic mead as a representative of that. We also have archaeological sources (the importance of glass vessels and drinking horns in high-status burials) and pictorial depictions (such as the possible valkyries on Gotlandic picture stones). Like today alcohol was used to produce social cohesion, it was a social drink mostly imbibed at feasts it seemed. Beer (or, more specifically ale) sees to have been the most widespread, followed by mead in regions that allowed beekeping and wine, which was imported from the south in barrels, as can be seen from finds in the big trading places. There are two hints of drug use in the Viking Age, however they are slight. I urge you to not use them just to confirm any biases you may have (which, unfortunately, is often the case in the history of narcotics) but to consider them on their own merits.

One is a grave from outside the circular fortress of Fyrkat in Denmark (Fyrkat Gr. 4) which contained a female burial with certain unusual features which, according to Neil Price, may point towards an identification of her as a sorceress. It also contained a large amount of henbane-seeds. Henbane is a narcotic and hallucinogen. However it is also known from later medieval sources to have been used in witches' potions. It may or may not have been used as a drug, then.

Even more ephemeral is the evidence from Gokstad, where a few seeds of Cannabis were found in the famous ship burial. They were probably originally contained in a small pouch. This might point to "personal use". However, I think it is as likely that it was a symbolic gift of hemp associated with the burial. As you can see, this evidence is by no means conclusive and I find it striking that the later sources we have mention no drug use except for alcohol.

From /u/Wee_little_puppetman