I can't speak for interactions with Spiro, but I can provide some more general info on magnesium.

A fair chunk of people are magnesium deficient. Wikipedia says upwards of 15%, and cites decreased intake as the reason. Basically, Americans (at least), aren't eating their vegetables.

Magnesium is generally excreted as waste if you intake more than the body needs. A common way to test for deficiency is through what's called "intravenous loading." In other words, the doctor injects a bunch of magnesium into your blood. If you absorb 20% or more of it, you're deficient. Under normal circumstances, overdose is extremely rare and usually only happens in people with decreased kidney function. In the case of overdose, it can be dealt with through calcium supplementation and stopping the source of the overdose (which is usually supplements that have been taken in a manner other than directed or a larger than usual dosage, or other non-food source).

Magnesium is required as part of the process of making DNA and RNA, as well as crucial in the function of numerous other enzymes in the body. In short, it's essential to pretty much every cell in the body. It helps a lot of people lose weight, because it improves their body's overall function and helps balance their electrolytes, which were probably out of balance to some degree.

From what I can find, Spiro is a magnesium-sparing diuretic. This means that, unlike some other diuretics, it does not cause a large loss in magnesium. It's also potassium sparing. However, from what I could find, the greater risk is still hypOkalemia and hypOmagnesemia (magnesium and potassium deficiency) in patients without impaired kidney function.

In fact, the brand-name drug Aldactone includes magnesium stearate in its formula, I surmise to help prevent magnesium deficiency. ( http://www.drugs.com/pro/aldactone.html )

Given that, I'd say you could probably take a magnesium supplement if you feel you need it. If you're unsure, talk to your doctor and see about getting tested. The odds of it building up in your system are unlikely, due to the efficiency by which the kidneys get rid of excess. However, don't take it just because you think it will help you lose weight. If it helps, it's because you're deficient, not because it's some kind of "weight loss supplement."

Also, if you're worried about overdose for whatever reason, then just increase your intake of leafy green vegetables, pumpkin seeds, sunflower seeds, and/or cashews, which are naturally abundant sources of magnesium, and from which it's much harder to overdose on magnesium compared to supplements.

Also, beware of taking too high a dose at once (150-250 is the dose I was told is normal for women - I was rxed 500). Magnesium is a major component of the colon cleansing liquid you take in the 24 hour period before a colonoscopy to help clear things out. I took this for maybe three days in a row, and ended up messing myself in my sleep. I would much recommend getting this through food if possible, as at least then your body seems to know what to do with it.