You are under 23 years old, diagnosed with cancer (there is some disagreement on this one), pregnant or lactating women, males with prostate disorders or high PSA values. I can tell you in my personal experience with clients over the years with many of these issues, there really doesn't seem to be a problem with cancer or prostate disorders but I mention it here because I have seen some literature that suggests potential problems, so do your research. There have been some reports of herpes flare-ups (i.e. cold sores) while on L-Arginine as well, but again, I have not seen this with my clients. There are no well-known drug interactions with L-arginine other than nitrate drugs (i.e. nitroglycerin) or vasculogenic drugs (i.e. Viagra); these could vasodialate blood vessels too much or drop blood pressure too low, although there have been no reports of this adverse event.