Because nitrate can lower blood pressure (see What are nitrate’s main benefits?), taking a nitrate supplement might cause an excessive drop in blood pressure in people who take blood-pressure-lowering medications. Similarly, nitrate may also interact with drugs that cause vasodilation (e.g., drugs used to treat heart failure and erectile dysfunction).[1][2] Always consult your doctor before taking a nitrate supplement if you use any of these types of medication.
Nitrate may pose a health risk to infants and young children, because a high intake can cause methemoglobinemia, a disorder where hemoglobin cannot transport oxygen effectively.[3][4][5] High levels of nitrate intake during pregnancy have also been associated with heart defects in newborns.[6]
Nitrate in the naturally occurring levels found in vegetables is likely safe for consumption, as is the intake of nitrate at levels typically found in dietary supplements. Furthermore, nitrate-rich vegetables probably have nitrate-induced health benefits. However, due to the increased use of nitrate salts (ammonium nitrate, sodium nitrate, potassium nitrate, calcium nitrate, etc.) in fertilizers, excessive nitrate levels are being found in vegetables and the water supply.[7] High levels of nitrate salts are also used as food additives to manufacture processed meats.[8][9] This creates an issue, because excessive nitrate intake can increase the levels of N-nitroso compounds (NOCs) in the body. Both high NOC levels and a high dietary nitrate intake have been associated with an increased cancer risk.[10][11][12][13][14] However, the relationship is complicated, so please read Does nitrate cause cancer?) for further details.
Studies that have examined the effect of nitrate/beetroot juice on exercise performance have done so when participants are well rested and not fatigued.[15][16][17][18][19] Therefore, it is not currently known whether nitrate/beetroot juice has the same effect on performance when participants are fatigued (i.e., when participants complete a performance test at the end of a tiring exercise bout). It is also important to note that, in general, the evidence underpinning the benefits of nitrate on exercise performance has a risk of bias because the studies have small sample sizes, sometimes lack a control group, and were funded/authored by manufacturers of the supplements/foods containing nitrate.[15][16][17][18][19]
While adverse effects of nitrate intake are uncommon, some people who drink beetroot juice as a source of supplemental nitrate can develop beeturia (temporary beetroot-colored urine), which occurs due to pigments called betacyanins in beetroot.[20] While unusual, beeturia is a harmless (and fun) side effect of consuming beetroot.