An example is glutathione, which always works much better when taken on an empty stomach
Katagiri R, Yuan X, Kobayashi S, Sasaki S
GHK-Cu - Copper Tripeptide Research Compound GHK-Cu Copper Tripeptide Research Compound GHK-Cu is a copper-binding tripeptide complex commonly studied for its role in peptide-metal signaling, extracellular matrix activity, and structural protein dynamics
She has been cited in The New York Times
Possible Side Effects Observed in Research & Experimental Settings Glutathione has been explored in laboratory and experimental research settings, and observations in some research contexts have included: Administration-site irritation Temporary redness or discomfort Headache-related observations Nausea-related observations Fatigue- or lethargy-related observations Dizziness-related observations Gastrointestinal-discomfortrelated observations Temporary flushing or warmth-related sensations Responses may vary depending on experimental conditions, research amount, administration frequency, and experimental protocol design