For most men, 4,000iu daily. That's the dose that consistently moves blood levels into optimal range without requiring a prescription or a doctor's supervision.
The NHS recommends 400iu. That number is designed to prevent deficiency at the population level. It's not designed to optimise your health. There's a difference between not deficient and optimal, and it matters.
Why the standard advice undershoots
400iu will stop you getting rickets. It won't get your levels into the range where vitamin D actively supports testosterone, immune function, mood, and muscle recovery. Multiple studies show that 2,000 to 4,000iu daily is what's needed to reach and maintain that range for most men.
Your body stores vitamin D as fat-soluble, so consistent daily dosing is more effective than intermittent high doses. Small amounts every day, not one large dose once a week.
Is 4,000iu safe?
Yes. The NHS states that taking up to 4,000iu daily is unlikely to cause harm. Toxicity from vitamin D is possible but requires sustained intake at levels significantly above that, typically 10,000iu daily for months. For most men, 4,000iu is the sweet spot.
If you're concerned about your levels, a standard vitamin D blood test will tell you where you are. Target range is generally 100 to 150 nmol/L. Most men come back well below that.
When to take it
With food that contains fat. Vitamin D is fat-soluble and absorbs better with dietary fat present. Morning or midday works well. Evening is fine too, though some men find it affects sleep at higher doses.
Take it daily. Consistency matters more than timing.
Related reading: Are most men in the UK vitamin D deficient?, Does vitamin D affect testosterone in men?