Reddit reviews Jewelry Scale,TBBSC Smart Weigh High Precision Digital Milligram Scale(50gx0.001g) (White)

Yep, that's what I thought. That's 50mg and 90mg respectively. You're lucky you made that mistake with Memantine and not something more potent and dangerous. .5mg (or 500 micrograms) would be equivalent to a few grains of salt crushed into a powder and would be impossible to measure on a scale that can only weigh amounts down to .01g (10mg).

You should really get a .001g scale to weigh out compounds like this, as that kind of scale isn't going to be very accurate at anything below 100mg. This is a good one for if you're on a budget, I used this scale for years and it's accurate down to 10mg, anything below 10mg will be somewhat inaccurate, though: https://www.amazon.com/PoLLux-Precision-Portable-MilliGram-Digital/dp/B06XWBH633/ref=sr_1_3_sspa?s=kitchen&ie=UTF8&qid=1539560773&sr=1-3-spons&keywords=milligram+scale&psc=1

This is a somewhat better one if you don't mind spending a little more money, it lets you adjust how it sits so you can keep it level which is a nice feature: https://www.amazon.com/Jewelry-Precision-Digital-Milligram-50gx0-001g/dp/B07DLKVL6X/ref=sr_1_4_sspa?s=kitchen&ie=UTF8&qid=1539560773&sr=1-4-spons&keywords=milligram+scale&psc=1

Remember that when you set it to measure grams, it's displaying the weight in grams, not milligrams, so anything that's to the right of the decimal point is your weight in milligrams. There are 1,000mcg in 1mg, 1,000mg in 1g, and 1,000g in 1kg and so on. So if you have a display that's saying 0.05 and it's set to weigh grams, that equals out to 50mg. 5mg would be .005g and .5mg wouldn't register on milligram scales and would require a microgram scale.