Heavy smokers could actually increase their chance of developing lung cancer if they are taking over 20 mg per day of β-carotene supplements.
That was the conclusion of two scientific papers published back in 2010. This was surprising as previous publications had found correlations between increased β-carotene serum levels and decreased cancer incidence in humans. The European Food Safety Authority (EFSA) set up a panel to investigate and have concluded that up to 15 mg per day have not been reported to have adverse health effects on the general population including heavy smokers.