Caffeine and Iron
Each entry’s own grades, side by side. A grade says how strong the evidence is for one outcome — it does not rank one entry over the other.
Compound or practice
Caffeine
The most-used and best-evidenced cognitive and physical stimulant on earth — precise dosing and a hard afternoon cutoff are the whole skill.
Grade A4 graded outcomesrevised Jul 10, 2026
A structured monograph in Prometheus's bundled library. Its grades are the monograph's own reading of the evidence, not a peer-reviewed review.
Compound or practice
Iron
Only supplement it against a ferritin number — transformative in deficiency, pro-oxidant and constipating in excess.
Grade A3 graded outcomesrevised Jul 10, 2026
A structured monograph in Prometheus's bundled library. Its grades are the monograph's own reading of the evidence, not a peer-reviewed review.
Graded in both
Outcomes are matched by name. Two entries can describe the same outcome in different words, so one listed further down may still be graded in the other.
No outcome has the same name in both entries. They may still overlap under different wording — read the two lists below.
Graded only in Caffeine
- Alertness / reaction time Moderate effectEvidence grade A
- Endurance performance Moderate effectEvidence grade A
- Strength / power Small effectEvidence grade B
- Sleep (afternoon dosing) Moderate effectEvidence grade A
Graded only in Iron
- Iron-deficiency anemia Large effectEvidence grade A
- Fatigue with low ferritin (non-anemic) Moderate effectEvidence grade B
- Restless legs syndrome Moderate effectEvidence grade B
What each entry says about the other
- Caffeine’s entry · Mind the timing: Cuts non-heme iron absorption ~40% — separate by 2 hours.
- Caffeine’s entry · Watch with: Cuts non-heme iron absorption ~40% — separate by 2 hours.
- Iron’s entry · Mind the timing: Coffee polyphenols cut non-heme iron ~40% — separate 2 hours.
- Iron’s entry · Watch with: Coffee polyphenols cut non-heme iron ~40% — separate 2 hours.
Both columns are read from the entries as they stand in this library. A comparison is not advice to take either one, and it is no substitute for a clinician who knows the person.