Many supplements are sold for stress, and few have trials that measured it. One has the clearest evidence, with a safety caveat worth knowing; two others are often recommended and held up less well when we read the trials. None of them is a substitute for addressing what is driving the stress, and if you feel anxious most days, or it is getting in the way of your life, that is a conversation for your GP rather than a supplement shelf. For everyday stress, here is what the evidence shows.
Ashwagandha
Ashwagandha is the most-studied herb here. In placebo-controlled trials of standardised extracts, people rated their stress lower after several weeks, and two pooled analyses agree on the direction; the larger of them rates its own certainty as low. Expect a moderate effect at most. The caveat: there have been rare reports of liver injury linked to ashwagandha, so it is best avoided if you have liver problems or take other medicines that affect the liver, and worth stopping if you notice anything like unusual fatigue, dark urine or yellowing of the skin. For most healthy people it may be reasonable to try; the point is to know the flag rather than to avoid it outright.
Magnesium
Magnesium is widely sold for stress, but the trials we cite did not measure stress: the one stress-related reading, a cortisol measure, sits inside a sleep trial. Two of its authorised health claims come close: it contributes to normal psychological function and to the normal functioning of the nervous system. Neither is the same as easing stress. Stress and poor sleep tend to feed each other, and the sleep guide covers where magnesium fits there.
L-theanine
L-theanine, an amino acid found in tea, is often recommended for stress. When we read the trials, the one built to test it for an anxiety disorder found no difference from placebo, and the stress results in the main stress trial did not hold up between groups. It is very well tolerated and some people like it for winding down, but we do not count it as a stress compound.
An important note if you take an antidepressant
If you are on an SSRI or SNRI, some stress and mood supplements interact with it, and two (5-HTP and St John's Wort) are a genuine hard stop. Before adding anything for stress while on an antidepressant, read the SSRI guide and check the specific combination. The free checker will show you the call and the reasoning for any pair.