Psychedelic Adverse Effects and Contraindications

Urgent first

If someone has chest pain, trouble breathing, a seizure, extreme agitation or confusion, or may be in danger, call 911 in the US. For possible poisoning in the US, Poison Help is 1-800-222-1222 (see poison help by country and poisoning: what to do first). In the US, the 988 Suicide & Crisis Lifeline is reached by calling or texting 988. This page is education only; it gives no guidance on using these substances.

Quick answer

Classic psychedelics such as psilocybin and LSD can cause physical effects (headache, nausea, higher blood pressure and heart rate) and psychological effects (fear, anxiety, confusion). Serious harm is uncommon in controlled studies, but risk is not zero, and outcomes outside clinics can be worse. Evidence labels: established for the commonly reported short-term effects, insufficient evidence for long-term and rare outcomes, evidence of harm where case reports show it.

Adverse effects

Short-term physical effects. NIDA lists headache, abdominal pain, nausea or vomiting, high blood pressure, rapid heartbeat, trembling and diarrhea, typically mild or moderate but sometimes needing medical care. [1] A 2026 review of supervised studies found average rises of about 19 mmHg in systolic and 8.7 mmHg in diastolic blood pressure, peaking about 60 to 90 minutes after dosing and generally resolving within 4 to 6 hours. [2]

Trial adverse events. In a phase 2 depression trial, adverse events occurred in 77% of participants, including headache, nausea and dizziness, and suicidal thoughts, behavior or self-injury occurred in all dose groups. [3] In another trial psilocybin brought more overall and more severe adverse events than placebo, with no serious treatment-related events. [4] The Cochrane review of trials in life-threatening illness found no treatment-related serious adverse events, with common mild to moderate effects of raised blood pressure, nausea, anxiety, emotional distress and psychotic-like symptoms that resolved when drug effects wore off or within a week. [5]

Frightening experiences ("bad trips"). NIDA says psychedelics can produce fear, anxiety and confusion, along with intense happiness. [1] In a survey of almost 2,000 people who took psilocybin outside a medical setting and had a challenging experience, 11% said they had put themselves or others at risk of harm. [1] Impaired perception can lead to dangerous behavior and injuries, especially if no one else is present who can help. [1]

Persisting perception changes (HPPD and flashbacks). NIDA describes a rare condition called hallucinogen persisting perception disorder, with recurring perceptions from a past experience. [1] A 2026 chapter explains that HPPD differs from brief flashbacks because it persists, that it appears relatively uncommon, and that in most cases it settles within a year, although a rare few need long-term treatment. [6] There are no evidence-based treatments yet; knowledge comes from case reports. [6]

Mental health. NIDA says there is some evidence psychedelics might trigger a schizophrenia-like illness in people with predisposing factors, but little evidence of long-term psychiatric problems for most people. It also says that in research settings with exclusions, the risk of psychosis or suicide is extremely low, while poor mental health outcomes may be more likely outside clinical use. [1]

Dependence. Limited research suggests psilocybin and LSD do not typically lead to addiction. [1]

Microdosing. Research has not established that microdosing is safe or effective. [1]

Products. Illicit drugs can contain contaminants such as fentanyl, which is colorless and odorless. [1] Natural mushroom products can vary in active content by more than an order of magnitude. [2]

Contraindications: who trials left out or flagged

"Contraindication" means a reason not to use something. For these substances, no approved product label was found in the sources read, so the best available information is who was screened out of studies or flagged in reviews:

• Psychosis or bipolar disorder, and sometimes other mental illnesses: excluded from modern trials as a precaution. [1]
• Mania, active substance use disorder, active suicidal ideation with intent: exclusions in one large trial. [4]
• Heart and blood pressure conditions: the review of cardiovascular safety says risk interpretation changes with heart disease, interacting medicines and repeated exposure, and that general reassurance is not supported. [2]
• Interacting medicines: see drug interactions and toxicology.
• Pregnancy: effects are not well understood. [1] A 2026 scoping review found no eligible human pregnancy outcome studies for psilocybin or ayahuasca, and warns that missing data is not evidence of safety. [7]

Why trial results do not transfer

Supervised studies use screened participants, standard products and staff. See psychedelic clinical trials and research limits. Anyone with a history of mental illness, heart disease, or who takes medicines should speak with a clinician about their situation: talking with a health professional.