Asklemmy

There is no universal supplement that everyone needs. There may be things people are prone to being short on based on gender, age, region, medical conditions, etc, but always ask your doctor first. They can run a full panel blood test to measure all your vitamin/mineral levels.

Many supplements negatively interact with prescription medications, or can cause health issues at high doses. Make sure that any supplements you take - including multivitamins - are on your medical record and verified against your new/existing prescriptions.

Cis women are more likely to need supplements. A lot of vitamins are lost because of periods, leading to chronically low iron and magnesium. People with intestinal disorders like IBS may struggle with maintaining potassium levels. But you wouldn't want to give someone with IBS a magnesium supplement without a counter treatment. Potassium is fat soluble, so you shouldn't take it unless you know you need it. And just because you're more likely to be low on something doesn't mean you are.

Bodies are weird. Every body is a little different and we're far from any kind of "universal" treatments. But if you're always feeling off it's worth a checkup. The vast majority of nutrition deficiencies are caused by underlying conditions, which requires treating BOTH the deficiency and the condition. But there isn't some magic vitamin that most people are missing.