L-theanine and Paraxanthine for Cognitive Improvement in Adults With ADHD and ASD

Inclusion Criteria: Adults (18+ years) diagnosed by a physician (per self-report) with ADHD or ASD \* Participants with a dual diagnosis and ASD and ADHD will be recruited but will only be included in the ASD group - their concurrent ADHD diagnosis will be included as an additional variable in exploratory analyses Exclusion Criteria: 1. Subjects with gross visual or auditory impairments that might limit their ability to perform neuropsychological tasks 2. Inability to read and follow written instructions 3. Physical, neurological, intellectual, or psychiatric impairments (except ADHD or ASD) that could affect cognitive and motor functions 4. Subjects on medications including antidepressants and antipsychotics that may affect performance in the tasks 5. Subjects on medications that are likely to interact with the administered substances, including regular intake of medication that could alter visual, auditory, cognitive, or motor functions (except stimulants) 6. History of head injury resulting in loss of consciousness/history of brain surgery 7. Subjects who have developed adverse effects when caffeine/paraxanthine /L-theanine/corn starch (will be in placebo) containing products were consumed 8. Subjects who are unwilling or unable to refrain from intake of L-theanine/ caffeine/ paraxanthine containing food or beverages within the 24 hours prior to each study visit 9. Intake of drugs containing caffeine, other phosphodiesterase inhibitors, or adenosine receptor blockers within the past 3 months 10. Intake of medications known to have pharmacological interactions with paraxanthine within the past 3 months 11. Current/past diagnosis of tics or other forms of dyskinesia 12. History of headache, drowsiness, anxiety, insomnia, or nausea following intake of caffeine or caffeine-containing beverages 13. Current/past history of smoking and/or alcohol or drug abuse 14. Subjects with absolute contraindications to undergo MRI after being screened by the TTNI safety screening sheet (Appendix) 15. Unwillingness or inability to entirely refrain from the use of electronic devices during study visits 16. Unwillingness or inability to follow written, on-screen, and verbal instructions given by the study team.