San Pedro effects and how long they last

Mescaline is a classic psychedelic, acting mainly on the serotonin 5-HT2A receptor like psilocybin and LSD. The effects listed by the Alcohol and Drug Foundation are:

• euphoria and increased energy
• perceptual changes, such as brighter light, intensified colour and geometric patterns
• a distorted sense of space and time
• difficulty concentrating, thinking and holding attention
• higher blood pressure and heart rate
• intense nausea and vomiting
• reduced appetite
• higher body temperature and sweating

The Basel study compared 500 mg mescaline with 100 µg LSD and 20 mg psilocybin in the same 32 healthy volunteers. At those doses the three substances produced comparable subjective effects on every questionnaire used; the authors found no evidence of a qualitative difference in the altered state itself. What sets mescaline apart is time: it takes longer to reach its peak and it lasts longest of the three (11.1 hours, against 8.2 for LSD and 4.9 for psilocybin).

The timings combine the Basel measurements with the ranges in the fact sheets above. They are averages for known doses of the pure compound. With cactus material the dose is unknown (see below), so the curve can be weaker, stronger, shorter or longer.

The slow start catches people out. The harm reduction organisation ICEERS notes that the effects of mescaline and San Pedro can take up to two hours to appear (ICEERS). People who decide after an hour that "nothing is happening" and take more can end up with a much stronger peak, arriving three hours later than expected. Wait at least two hours before even thinking about it.

For pure mescaline the ranges are reasonably well described. ICEERS gives 150 to 700 mg of mescaline hydrochloride as the active range, with around 100 mg as threshold, 200 to 300 mg as an average experience and 300 to 500 mg as a high dose. The Basel study used 300 and 500 mg.

For the cactus, nobody can say how much material contains a given amount. Measured mescaline content ranges from 0.053% to 4.7% of dry weight across the San Pedro group (Ogunbodede et al. 2010). At the low end you would need impossible quantities; at the high end a modest amount is already a strong dose. That is why this site gives no amounts of cactus, and why reports from other people are a poor guide to your own plant.

Beyond dose, the surroundings matter a great deal. In interviews with 151 healthy volunteers from six psychedelic studies in Basel, music and access to nature were the setting factors most often linked to a good experience, while the sterile hospital room was the most common complaint. A trusting relationship with the person supervising was valued most (Ley et al. 2025).

Practical consequences of the long duration:

• Start in the morning. An afternoon start runs into the night.
• Clear the next day as well. Tiredness and headache are common in the 12 to 24 hours after.
• Have a sober person present for the whole period, not just the peak.
• Do not drive or cycle until the next day.

Composite account, assembled from patterns that recur in published experience reports and harm reduction information. It does not describe one real person.

Hour 0 to 1. A Saturday at 9 in the morning, at home with a sober friend. The bitter taste is the first hurdle. Within 40 minutes the stomach turns heavy and the nausea arrives; around the hour mark there is one bout of vomiting, after which the body feels lighter.

Hour 1.5 to 3. The light in the room warms up. Wood grain on the table seems to slide, very slowly. Music sounds layered, every instrument in its own place. There is a strong urge to be outside, and a short walk in the garden feels long and detailed.

Hour 3 to 7. The peak. Closed eyes show bright geometric patterns. Thoughts move from memory to memory, some warm, some sad, without a clear thread. Time is hard to track; an hour of lying in the grass feels like ten minutes, and ten minutes of waiting for tea like an hour. The body feels warm, the heart beats faster than usual.

Hour 7 to 11. A long, calm descent. Conversation becomes possible again and is mostly about small things. Food is uninteresting until evening. Colours stay brighter than normal well into the dark.

The next day. Sleep comes late. The morning brings a dull headache and tiredness that lasts until the afternoon.

The recurring elements in reports like this are the nausea in the first hour, the late peak, the length of the plateau and the tired day after. Accounts of difficult sessions tend to share a second dose taken too early, a setting with strangers or noise, and no one sober nearby.

Before any session, read nausea, risks and safety. For the chemistry and the plants, see what San Pedro is. For the legal position, which in most countries makes preparing the cactus an offence, see the legal overview.