Vitamin doses: What’s safe and effective?

Just like with medications, vitamins have appropriate doses. Too little of a dose and it probably will have little effect. Too much of a specific vitamin may or may not increase efficacy, but does increase the risk of side effects.

With that said, however, you can safely manipulate vitamin dose within reason— and usually gain in effectiveness— without causing side effects.

Vitamin D: It’s Really Not about Dosing

Vitamin D is a fat-soluble nutrient. This means that it can build up in your tissues and blood. Too much vitamin D in your system can have negative effects, such as an increase in blood calcium, gastrointestinal problems, kidney stones and even heart arrhythmias.

With vitamin D then, the most important number to know is your blood level. Once you know your baseline blood level, an appropriate daily dose can be established and, if needed, increases in the dose can safely be manipulated for short periods of time.

This works because it’s been established that for every 1,000 international units (IU) of vitamin D, you can expect your blood level to increase by approximately 10 ng/ml. Now, this relationship between dose and blood level is not set in stone, but it holds true for most people.

Mainstream medicine believes that a healthy vitamin D blood level is around 36 to 40 ng/ml. However, other organizations, like the Vitamin D Council, believe 50 to 80 ng/ml is optimal.

Using the Vitamin D Council’s range as the upper limit allows you some wiggle room. If your vitamin D blood level is, say, 30 ng/ml and your daily dose is 1000 IU, then potentially you could increase  your dose during flu season to 3000 IU a day — expecting your blood level to increase to around 50 ng/ml (adding 2000 IU should bump up your blood level around 20 points).

But you really can’t do any of this without knowing your baseline vitamin D blood level. So make it a habit to get tested every year before the flu season begins. This way you can manipulate your vitamin D dose in a reasonable and conscious manner.

By the way, there’s little evidence to suggest that increasing vitamin D dose when you think you’re coming down with the flu will help to fight off the virus. However, there is evidence suggesting that vitamin D can promote antimicrobial responses to pathogens and help regulate the activation of B and T cells. This may help to prevent infections.

Vitamin C: Easier to Manipulate

Vitamin C is a water-soluble nutrient, which means that it doesn’t build up to any significant degree in blood and tissues. So increasing vitamin C dose when you don’t feel well or even throughout the entire cold and flu season is safe and effective. As a matter of fact, one animal study showed that vitamin C could eliminate the flu virus from the lungs. Another showed that vitamin C is an effective treatment against the flu strain H3N2.

Mainstream medicine uses around 100 mg a day as the standard dose. However, higher doses for a short period of time are safe. The upper dose limit for vitamin C is 2000 mg a day. Absorption decreases and watery stools can develop if you increase your dose above the upper limit.

What You Need to Know

Yes, you can take too much. But that’s true for medications as well as supplements. But using vitamin D blood level and the upper limit of vitamin C doses gives you some room to increase dose during an illness or throughout the cold and flu season.

Michael A. Smith, M.D., is the senior health scientist and online personality for Life extension, the world's leading organization dedicated to extending the healthy human life span. He is an author of “The Supplement Pyramid” and hosts Healthy Talk radio on A graduate of the University of Texas, Southwestern Medical Center in Dallas, Texas, Dr. Smith completed an internship in internal medicine at the University of Utah and three years of a residency in radiology at UT Southwestern Medical Center.