Muscle cramps are invariably related to circulatory problems, but that offers a very wide range of causes and cures, and some of the cures can be quite dangerous - especially if carried to extremes.

As noted, quinine sulphate is an "instant cure" for large muscle cramps; but was banned in the US as an over-the-counter (OTC) drug a few decades ago and is now available (in useful amounts) only by prescription. The cited reason for the ban was that quinine is a specific relaxant at the tendon/muscle interface where cramps are "triggered," but this is the same "critical area" where muscle function signalling occurs so it can also affect heart rhythm. Quinine also may be less effective for "small cramps" like those in toes and fingers.

Postural peculiarities can cause local reduction in circulation in a particular muscle, and even a fairly brief restriction in flow can cause the initiation of a cramp. Some people become fairly "active" during deep (REM) sleep, and "act out" their dreams, but others may become almost "rigid" and the prolonged holding of one position may allow a restriction of flow to a localized part of the body - particularly a part of a leg - to "accumulate a circulation deficit" leading to a cramp. (The small bar of soap gives you something to "kick around" and may lessen the likelihood that you'll hold a fixed position for long periods? Sleeping after imbibing also tends to reduce "sleep-motions" making one more prone to localized blockage of circulation?)

Sleep disturbances caused by things like apnea or COPD are commonly thought of in terms of oxygen depletion, which can cause cramps; but also can affect CO₂ levels in the blood that can cause the entire breathing and circulation process to "turn to crap." For one particular kind of cramp that I get, the most immediate fix is an 8 oz glass of saturated bicarbonate of soda to "kick up" the CO₂, but that doesn't work for the other kind(s). Long-term or habitual use of large doses of bicarb can cause unwanted side effects, but my team of advisors seem undisturbed by my reports of occasional use. It does include a fairly massive slug of sodium, but that isn't a concern for me.

Recent opinion on reduced sodium intake have found that not all people with high blood pressure benefit from drastic reduction. Some people are extremely sensitive to sodium, and need to restrict it; but for others of us it seems to have very little effect on bp. You may be somewhere in between. Drastic reductions in sodium can also cause uncomfortable side effects (see the Yuppie Flu of a decade ago) so you need to find out whether, and to what extent, you personally are affected before deviating from standard dietary guidelines.

Many of the other suggestions such as Calcium, Potassium, Vitamins, and such are important for general health, but to be effective require the creation and maintenance of appropriate and stable long-term levels in the system, so few of them are of much use for the immediate treatment of a specific incidence of a cramp (IMO). Getting up to get the pill may relieve the sluggish circulation, which will help. Taking something with a glass of water could help if you're a bit dehydrated. Massage, movement, and stretching often are about the only things that do a lot of immediate good for some kinds of cramps. Use of a good "massage vibrator" on the affected muscle may be helpful, but I have the last real "vibrator" sold in the US. Since ca. 1948, there are no readily available devices that "vibrate." The current ones potentially useful on large muscles just "hum lightly" and are not very effective.

(Personal opinion based mainly on long-term observation of a sample of only one rather decrepit body.)

John