Ehn 1968.

Methods	Type of study: parallel, 3‐arm randomised controlled trial Country of study: Sweden Study setting: single centre Number of participants randomised: not reported Number of participants analysed: (after 4 donations): Treatment arm 1: 12 Treatment arm 2: 12 Treatment arm 3: 18 Follow‐up time points: 2 months after 6 donations subsequent to baseline donation; inter‐donation interval of 2 months Hb threshold for deferral from donation: not reported Source of funding: supported by AB Hassle, Molndal Medical Research Fund, County of Ostergotland
Participants	Young male first‐time donor conscripts Mean age (years): both arms combined (subset of 58 subjects): mean 20, range 18 to 23 Sex (male/female): 100%/0%
Interventions	Treatment arm 1: oral iron (Ferromyn S, AB Hassle); 37 mg as ferrous succinate + 0.11 g succinic acid taken as 2 tablets twice daily for 2 weeks. Total dose: 2000 mg Treatment arm 2: oral iron (Ferromyn S, AB Hassle); 37 mg as ferrous succinate + 0.11 g succinic acid taken twice daily for 2 weeks. Total dose: 1000 mg Treatment arm 3: oral placebo taken twice daily
Outcomes	Haemoglobin; serum iron, total iron binding capacity; stainable bone marrow iron; sideroblasts; desferrioxamine test
Notes	—
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	The method of randomisation was not reported
Allocation concealment (selection bias)	Unclear risk	Allocation concealment was not reported
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Blinding of study participants was not reported although a placebo was used
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Blinding of outcome assessors was not reported
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Some withdrawals were described but the number of participants randomised to each treatment arm was not reported
Selective reporting (reporting bias)	Unclear risk	All outcomes listed in the manuscript were reported, but no study protocol was available to determine the full list of pre‐specified outcomes
Other bias	High risk	The study was supported by AB Hassle, manufacturers of iron supplements