Jacobson 2000Low.

Methods	Randomised, double‐blind, placebo‐controlled trial with parallel group design (two intervention groups).
Participants	Country: United States of America. Number of participants randomised: 121, mean age 42, 58% males. Inclusion criteria: adults 18 years of age and older who smoked one or more packs of cigarettes per day and were not currently taking the study vitamins. Exclusion criteria: nondetectable polycyclic aromatic hydrocarbon PAH‐DNA adduct levels in mononuclear cells and plasma vitamin levels higher than 1.0 mg/dl for vitamin C, 15 mg/dl for beta‐carotene, and 1.2 mg/dl for alpha‐tocopherol at the first study visit.
Interventions	Participants were randomly assigned to receive: group 1: vitamin C 500 mg, alpha‐tocopherol 400 IU, beta‐carotene 6 mg (n = 60); group 2: placebo (n = 61). Participants were supplemented and followed 0.5 years.
Outcomes	The primary outcome measure was: DNA damage.
Notes	Compliance with treatment was not reported. Seventy‐three participants completed the trial. Drop‐out rates were high in both groups, but were higher in the placebo (53%) than in the treatment (35%) group. Additional information received through personal communication with the authors.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Sequence generation was achieved using computer random number generation.
Allocation concealment (selection bias)	Low risk	Allocation was controlled by a central and independent randomisation unit, so that intervention allocations could not have been foreseen in advance of, or during, enrolment.
Blinding (performance bias and detection bias) All outcomes	Low risk	The trial was described as blinded, the parties that were blinded, and the method of blinding was described, so that knowledge of allocation was adequately prevented during the trial.
Incomplete outcome data (attrition bias) All outcomes	Low risk	The numbers and reasons for dropouts and withdrawals in all intervention groups were described.
Selective reporting (reporting bias)	Low risk	Pre‐defined, or clinically relevant and reasonably expected outcomes are reported on.
Other bias	Low risk	The trial appears to be free of other components that could put it at risk of bias.