The use of magnesium supplements to help with leg cramps is pretty well known and is documented as being of significant help in those cases where it's appropriate, but probably won't help in other cases.

Opionions that I've found from what appear to be reliable sources are that magnesium supplements are only helpful in cases where there is, or is very likely to be, a coexisting potassium defecit, and combining the two might reduce the amount of magnesium needed, if there's a significant difference in the cost of either.

Both are available OTC (over the counter - i.e. without prescription) in the US, and dosage withing the OTC package suggestions should be safe, however there are some hazards associated with "potassium overload" with extremes of excessive overdose. (I haven't seen similar warnings for magnesium, but "extremism" is seldom helpful.)

The only reliable way to know if you really will benefit from either is by serological tests (laboratory blood tests) that most physicians, here at least, will generally include in any annual physical if you report your symptoms properly.

John