what’s the difference between mg and mcg?
mg: milligram - one one thousandth of a gram
mcg: microgram - one millionth of a gram.
That is if you are talking about units of weight. An mg is also a car. 
Another thing to note is that GENERALLY SPEAKING if you are getting a substance in the miligrams that substance effects a gland or process which produces another substance, GENERALLY in much smaller quantities, and the byproduct substance is what you’re actually shooting for.
In the case of substances that come in the micrograms or very few milligrams, such as melatonin, you are dealing with a brain-level or perhaps blood-level chemical that does not necessarily need to creat a byproduct to start working.
So if you don’t know a lot about the substance, and it comes in 30 mcg capsules, do a little research first, because it’s probably a hard core little chemical. 
That doesn’t mean it’s unsafe, but it does merit your further study.
Conversely, buying a 500mg capsule of an OTC doesn’t make it safe, either.
I guess what I’m trying to say is that taking any substance for reasons other than what the label says can be risky… for that matter taking things IN ACCORDANCE with the label can be risky, so learn a little more before taking a bunch of drugs with impunity.
I’m not saying that’s what you’re going to do, but instead I’m just letting you know that the fact that something is herbal or ‘natural’ doesn’t make it inherently safe… I mean, arsenic is 100% natural… but it’s also 100% poisonous…
Oh well, I’ve unintelligibly ranted enough.