The bottom line

Both sit at the moderate evidence tier, but they earn it for slightly different things. Ashwagandha has the more direct stress data: randomized trials show meaningful drops in perceived stress and cortisol over roughly 6 to 8 weeks, plus modest sleep and anxiety gains, at 300 to 600 mg/day of root extract standardized to about 5% withanolides. Magnesium's evidence is strongest for sleep quality, muscle relaxation, blood pressure, and migraine prevention, with stress benefit most likely in people who are actually low in magnesium; typical dosing is 200 to 400 mg elemental/day, with glycinate or citrate absorbing best. Safety is where they diverge most: magnesium is very safe, its main issue being diarrhea at higher doses (use caution in kidney disease). Ashwagandha carries real cautions: avoid in pregnancy and breastfeeding, it commonly causes GI upset and drowsiness, may aggravate autoimmune thyroid disease or interact with sedatives, thyroid meds, and immunosuppressants, and rare but serious liver injury has been reported. Pick ashwagandha if targeted cortisol and anxiety reduction is the goal and you have none of those cautions; pick magnesium if you want a low-risk option, suspect you are deficient, or your main issue is sleep and muscle tension. This is educational, not medical advice.