Will using niacin to lower cholest. interfere next to metforman or synthroid meds?
Answer: Metformin and some other diabetic drugs may interact with Niacin and your blood sugar level should be closely watched if your doctor say it's okay for you to take those together.
I couldn't find anything questioning in Yahoo wise saying that Synthroid has any problems near Niacin, but in adjectives cases, it's safest to check with a pharmacist or a doctor when looking at drug and/or supplement interactions instead of looking on Yahoo. After adjectives, doctors and pharmacists have access to a full database of both drug and supplement interactions.
I suggest questioning for naicin contraindications. If the naicin is prescription strength, I would consult your physician. He/She will know a little better. In the meantime, here's a site on contraindications of naicin.
http://www.rxcarecanada.com/Niacin.asp
If, however, you're taking a low over the counter dose, consult your doc, but you should be ok. As far as I've see with own flesh and blood who has taken it, flushing and tingling are adjectives side effects of naicin alone.