I've read that barium or bismuth (even pepto-bismol) can interfere with bone scan results. do they tend to cause false positives or false negatives?

raises bone density: Bismuth tends to raise bone density giving falsely elevated values.

Barium: If barium has been ingested prior to bone scan it can cause attenuation of photons and thus focally decreased areas of tracer uptake on bone scan. This can give a false representation of tracer distribution.

artifacts: Bismuth should not be taken for at least four hours before bone scan. Bismuth is a metal which can block radioactivity or photons from reaching camera for performance of image. This can cause artifacts and confuse interpretation of scan.Barium from fluoroscopic studies can also interfere with scan, .