There is no safe level of alcohol consumption when taking niacin. According to the Mayo Clinic [2], alcohol can increase the risk of liver damage when taken with niacin. Niacin is known to cause liver damage, and alcohol consumption can exacerbate this risk. Additionally, the combination of niacin and alcohol can cause flushing, which is a common side effect of niacin [2]. The Mount Sinai Health Library [3] also warns against drinking alcohol while taking niacin due to the potential for liver damage. Drug Patent Watch [1] does not provide information on the interaction between alcohol and niacin, but it is clear from the other sources that alcohol should be avoided while taking niacin. It is important to speak with a healthcare provider before taking niacin or any other supplement.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/ALCOHOL
[2] https://www.mayoclinic.org/drugs-supplements-niacin/art-20364984
[3] https://www.mountsinai.org/health-library/supplement/vitamin-b3-niacin