Well this is an easy one. The answer is no. LSD is a synthetic drug, it did not exist before the 20th century.

So why would anyone think that? In the 60s and 70s as more refined drugs were being invented and recreational uses of drugs became more culturally prevalent, scholars started looking for parallels in the ancient world. The long unsolved question of what happened during the Eleusinian mysteries became the perfect target. (The Delphic oracle is another popular one.)

The Eleusinian Mysteries was an annual festival held at a sanctuary at the border of Attica. It was concerned with Demeter and her daughter Kore (Persephone) and the beginnings of the practice of cultivating wheat. You had to be ritually pure to participate, pay an entry fee, and come at least twice as you moved up the rungs from novice to initiate (called mystai – hence the word ‘mystery’). We know of a number of people who got in trouble for parodying or revealing the mysteries and the few hints that we have are tempting to scholars who want to fill in the gaps.

The Road to Eleusis, published in the 70s popularized the theory that ergot, a precursor to LSD, was part of the mysteries. However, this is pretty much universally dismissed by the serious academic community. Ergot poisoning is not LSD, it is VERY dangerous. It was known to the ancients as a deadly disease and was known particularly for poisonings so bad that they induced miscarriages, not exactly the kind of entertainment you find in a fertility cult. Others have suggested psychoactive mushrooms or an opiate since the poppy is a flower sacred to Demeter. We know opium was used in other parts of the ancient world, but there is no evidence for it in conjunction with this cult.

The much more likely answer is the good old opiate of the masses – religion. There is absolutely nothing in our evidence to suggest that there was any kind of drug being used other than the power of a communal experience. The Eleusinian Mysteries played a big part in constructing Athenian social identity at the height of the city’s power. Rather than a drug trip, a stirring political rally might be the better parallel. Burkert’s Ancient Mystery Cults, though dated in some ways, makes this point relatively convincingly. If you are interested in learning a bit more about mystery cults, Cosmopoulos’ Greek Mysteries: The Archaeology of Ancient Greek Secret Cults has some very good essays.

I think as moderns its sometimes easy to forget how different our physical experience of the world is from the ancients. The Greeks had great trust in pharmaka and we have some of the ancient recipes for medicine and magic. While there are traces of things with real effects in these recipes, they mostly rely on the great power of the placebo. We live in a world where we have much greater control and understanding of plants, chemicals, etc. The ancients couldn’t pop an aspirin or get surgery and fix their suffering, they had nothing like the highly refined drugs that we have. Even the ones that might seem cruder (like cannabis) existed only in much less potent forms in the ancient world.