Most people searching this question want to know about LSD, the psychedelic drug commonly called “acid.” LSD produces intense changes in perception, mood, and thought that last 8 to 12 hours, with effects beginning within about 30 to 60 minutes of taking it. The experience ranges from visual hallucinations and heightened emotions to physical changes like increased heart rate and dilated pupils. Here’s what happens in your brain and body, how long it lasts, and what the risks look like.
How LSD Affects Your Brain
LSD works primarily by binding to serotonin receptors in the brain, specifically one called 5-HT2A. Serotonin is a chemical messenger involved in mood, perception, and cognition. When LSD latches onto these receptors, it activates signaling pathways that normal serotonin activity doesn’t trigger, which is why the experience feels so different from an ordinary mood shift. LSD also interacts with dopamine and adrenaline receptors, contributing to its wide-ranging effects on how you think, feel, and process sensory information.
One of the most measurable brain changes is a dramatic increase in connectivity between regions that don’t normally communicate much. The primary visual cortex, for instance, starts talking to parts of the brain it usually ignores. This cross-wiring correlates directly with the hallucinations people report. Brain entropy, a measure of how unpredictably neural signals fire, also increases globally during a trip.
What a Trip Feels Like
The psychological effects of LSD are wide-ranging, but disordered cognition is considered a more fundamental feature than any single mood change. Your normal mental filters loosen. Things that seemed meaningless before can suddenly feel loaded with significance. In controlled studies, LSD caused people to rate previously neutral music excerpts as deeply meaningful.
The most commonly reported experiences include:
- Visual changes: patterns, colors, geometric shapes with eyes open or closed, and sometimes full scenes or imagery from the past
- Synesthesia: senses blending together, such as “seeing” sounds or “hearing” colors
- Emotional amplification: increased feelings of happiness, closeness to others, openness, trust, and wonder
- Altered sense of self: feeling detached from your body or from reality itself, often experienced positively rather than as panic
- Changed sense of meaning: ordinary objects or experiences feeling profound
The overall mood during a trip is positive for most people in controlled settings. Researchers who observed subjects after a 200 microgram dose rated them as happier and more detached from reality, without significant increases in anxiety or paranoid thinking. That said, the experience isn’t always pleasant. Some people experience cognitive disorganization, delusional thinking, or intense fear, particularly at higher doses or in uncomfortable environments. These negative experiences are what people refer to as a “bad trip.”
Physical Effects on Your Body
LSD doesn’t just change your mind. It ramps up your sympathetic nervous system, the same system responsible for a fight-or-flight response. In clinical studies, LSD significantly increased blood pressure, heart rate, body temperature, and pupil size. It also raised levels of cortisol (a stress hormone), adrenaline, and oxytocin (often called the bonding hormone).
Dilated pupils are one of the most visible signs someone has taken LSD. The rise in body temperature can make you feel flushed or sweaty. Some people experience nausea, jaw clenching, or tremors. These physical effects generally follow the same timeline as the psychological ones, peaking a few hours in and gradually fading.
How Long It Lasts
LSD has one of the longest durations of any common psychoactive substance. Effects typically begin within 30 to 60 minutes, though some people notice changes as early as a few minutes after taking it. The experience peaks around 2.5 to 3 hours in.
At a moderate dose (100 micrograms), the total effect lasts an average of about 8 hours, with a range of 5 to 14 hours. At a higher dose (200 micrograms), that average stretches to about 11 hours, and some people feel effects for up to 19 hours. Residual effects like subtle mood changes or difficulty sleeping can linger for 12 to 48 hours after the main experience ends. LSD’s plasma half-life is roughly 3 hours, meaning the drug is cleared from your blood relatively quickly, but its grip on serotonin receptors outlasts its presence in the bloodstream.
Longer-Term Risks
LSD is not considered physically addictive, and lethal overdoses are extraordinarily rare. The primary risks are psychological.
The most discussed long-term concern is hallucinogen persisting perception disorder, or HPPD, a condition where visual disturbances from the trip continue days, weeks, or even months afterward. These can include trailing images, halos around objects, or patterns in peripheral vision. HPPD is rare and more often diagnosed in people with a history of other psychological conditions or substance use, but it can occur in anyone, even after a single dose. Because prevalence is low, HPPD frequently goes unrecognized by clinicians.
A bad trip can also leave a lasting psychological mark. Intense fear, paranoia, or panic during the experience can be genuinely traumatic, particularly for people with underlying anxiety disorders or a predisposition to psychosis. LSD does not cause psychosis in the clinical sense for most users, but it can trigger or worsen psychotic symptoms in vulnerable individuals.
LSD in Clinical Research
LSD is being studied as a potential treatment for depression, anxiety, and other psychiatric conditions. In a 2025 randomized trial of patients with moderate-to-severe major depression, those who received therapeutic doses of LSD alongside supportive psychotherapy showed meaningfully greater reductions in depression scores compared to a control group receiving very low doses. The improvements persisted through 12 weeks of follow-up. The results were promising enough to support moving into larger trials, though they’re not yet definitive.
Earlier research found that LSD can induce what participants describe as mystical or transcendent experiences, and that a single session can increase personality trait openness (a measure of curiosity, creativity, and willingness to try new things) for at least two weeks afterward. These findings have driven growing interest in psychedelic-assisted therapy, though LSD remains a controlled substance and is not approved as a medical treatment.
Other Meanings of “Acid”
Stomach Acid
Your stomach produces hydrochloric acid with a baseline pH of roughly 1.5, making it extremely acidic. This acid serves two main purposes: breaking down proteins so your body can absorb nutrients, and killing bacteria and other microbes in your food before they can colonize your gut. Without this acid barrier, your intestines would be far more vulnerable to food-borne infections.
Acid and Your Teeth
Acidic foods and drinks erode tooth enamel through a purely chemical process, no bacteria required. Drinks with a pH of 5.5 or lower soften and dissolve the enamel surface over time. Carbonated soft drinks typically fall between pH 2.3 and 3.4, fruit juices between 2.1 and 3.6. Calcium in your diet helps counteract this by supporting the natural cycle of mineral loss and repair that teeth go through constantly.
Corrosive Acids on Skin
Strong industrial or chemical acids cause burns on contact with skin, eyes, or mucous membranes. If a corrosive acid touches your skin, flushing the area immediately with large amounts of water is the standard first-aid response. For eye contact, continuous irrigation with water for at least 15 minutes is recommended. Clothing that has been soaked through should be removed immediately to prevent the acid from continuing to damage the skin underneath.