Doctor prescribed 5,000 ui d3. i've tried 2 brands and both make me feel drained by the afternoon. it's like my muscles feel paralyzed. alternative?

I presume at your: age you are taking a vitamin D supplement because you have been diagnosed as being deficient. Otherwise, unless you have osteoporosis or some other unusual disease for a man your age, you shouldn't need this. If you are deficient, this is a reasonable, perhaps low, replacement dose and should not be causing notable side effects. Toxicity is usually reported at 40,000 IU or higher. Review with Doc