Per the Mayo Clinic website:
“Precautions
Use of beta-carotene has been associated with an increased risk of lung cancer in people who smoke or who have been exposed to asbestos. One study of 29,000 male smokers found an 18% increase in lung cancer in the group receiving 20 mg of beta-carotene a day for 5 to 8 years. Another study of 18,000 people found 28% more lung cancers in people with a history of smoking and/or asbestos exposure. These people took 30 mg of beta-carotene in addition to 25,000 Units of retinol (a form of vitamin A) a day for 4 years.
However, one study of 22,000 male physicians, some of them smokers or former smokers, found no increase in lung cancer. These people took 50 mg of beta-carotene every other day for 12 years. If you smoke or have a history of smoking or asbestos exposure, you should not take large amounts of beta-carotene supplements for long periods of time. However, foods that are rich in beta-carotene are considered safe and appear to lower the risk of some types of cancer and possibly heart disease.”
Beta carotene decreases the effectiveness of niacin which improves good cholesterol.