| Harmala alkaloid | Dangerous | Extreme serotonin syndrome risk; potentially fatal — MAOIs massively potentiate MDMA |
| PCP | Dangerous | — |
| Peganum harmala | Dangerous | Extreme serotonin syndrome risk; potentially fatal — MAOIs massively potentiate MDMA |
| Cocaine | Unsafe | — |
| 3-FMA | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| 4-MMC | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| 8-Chlorotheophylline | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Adrafinil | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Anandamide | Caution | Cannabis can unpredictably intensify psychedelic effects and increase anxiety |
| APICA | Caution | Cannabis can unpredictably intensify psychedelic effects and increase anxiety |
| Atropa belladonna | Caution | Unpredictable compounding of hallucinogenic effects with anticholinergic delirium |
| Benzydamine | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Bromantane | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Cannabidiol | Caution | Cannabis can unpredictably intensify psychedelic effects and increase anxiety |
| Datura | Caution | Unpredictable compounding of hallucinogenic effects with anticholinergic delirium |
| Dichloropane | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Diphenhydramine | Caution | Unpredictable compounding of hallucinogenic effects with anticholinergic delirium |
| Ephedrine | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Hexedrone | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Methcathinone | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Nicotine | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Oxiracetam | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Phenylpiracetam | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Rhodiola Rosea | Caution | Increases anxiety, cardiovascular stress, and psychological intensity |
| Cannabis | Uncertain | — |
| MDMA | Uncertain | — |
| Mescaline | Uncertain | — |
| 1,3-Butanediol | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| 25E-NBOH | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| 2C-T | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| 2C-T-2 | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| 2C-T-21 | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| 2C-T-7 | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| 3-Cl-PCP | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| 3-HO-PCE | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| 3-HO-PCP | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| 3-MeO-PCE | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| 3-MeO-PCMo | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| 3-MeO-PCP | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| 4-MeO-PCP | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| Allylescaline | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Ayahuasca | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Deschloroketamine | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| DET | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Diphenidine | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| DiPT | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| DOM | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| DPT | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Efavirenz | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Ephenidine | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| EPT | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| HXE | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| Ibogaine | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Inhalants | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| Memantine | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| MET | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Methallylescaline | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| MiPT | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| MPT | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| MXiPr | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| O-PCE | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| PCE | Low Risk & Synergy | Produces unique synergistic effects; often combined |
| Psilocybin Mushrooms | Low Risk & Synergy | Cross-tolerance exists; effects compound |
| Lorazepam | Low Risk & Decrease | Depressants dull psychedelic effects; benzodiazepines are commonly used as trip-killers |