I recently visited several offline pharmacies in different cities and found that high-dose vitamin D (at least several thousand IU) was almost nonexistent (although you could take multiple tablets at once). This is likely because long-term use of low doses wouldn't exceed the recommended daily intake. However, according to guidelines, my blood test revealed a severe vitamin D deficiency, suggesting a high-dose regimen for 8 weeks (more than 5000 IU daily) as a correction period, followed by a maintenance period of around 1000 IU. However, it still depends on individual dietary habits and absorption rates, and regular checkups are necessary for confirmation. But members of the Biohacker group generally reported that even with high doses, it's difficult to exceed the recommended daily intake; many tried to increase the dose further but to no avail. In any case, this level of deficiency is likely widespread (severe) for many people, especially those without consistent outdoor activity. A blood test to confirm this and a doctor's advice are recommended.