Slow-Mag is a magnesium supplement in a slow-release form. It is taken for muscle cramps, magnesium deficiency, and by people on medicines that deplete magnesium. The slow-release design is intended to reduce the diarrhoea magnesium commonly causes.
What is Slow Mag?
Slow-Mag is a magnesium supplement, sold in South Africa as a general-sale product rather than a scheduled medicine. Check your pack for the exact magnesium salt and elemental content, as formulations in this range differ and some include calcium.
Magnesium is a mineral involved in over 300 enzyme reactions. Its most relevant roles here:
- Muscle and nerve function — magnesium opposes calcium in muscle contraction, so a shortage leaves muscle more excitable and prone to cramping.
- Energy production, as a co-factor in ATP metabolism.
- Heart rhythm and blood pressure regulation.
- Bone structure — most body magnesium is in bone.
- Blood sugar regulation.
The “slow” in the name is the point. Magnesium salts draw water into the bowel — which is exactly how magnesium-based laxatives work. A slow-release formulation spreads absorption out, reducing the laxative effect that makes ordinary magnesium supplements hard to tolerate.
The salt matters for absorption. Oxide is cheap but poorly absorbed and most laxative; citrate, glycinate, chloride and lactate are absorbed better and are gentler. If a magnesium supplement has given you diarrhoea before, the salt is usually why.
What is Slow Mag commonly used for?
- Muscle cramps, particularly night cramps in the legs — the commonest reason people buy it.
- Diagnosed magnesium deficiency.
- Alongside medicines that deplete magnesium — proton pump inhibitors taken long term, and diuretics such as furosemide.
- Muscle twitching and restless legs.
- Constipation, where the laxative effect is the aim rather than a side effect.
- Migraine prevention, where magnesium has some supporting evidence.
- Premenstrual symptoms.
- In alcohol dependence, where deficiency is common.
- Poorly controlled diabetes, where magnesium is lost in urine.
Be honest about cramps. Magnesium is widely taken for night cramps, and the evidence that it helps in people who are not deficient is weak — trials in older adults have generally not shown benefit. It does help where deficiency is genuinely present.
Cramps have other causes worth considering: dehydration, medicines (diuretics, statins), thyroid disease, peripheral arterial disease, and in pregnancy. Cramps with weakness, numbness or wasting are not ordinary cramps and need assessment.
Magnesium deficiency is under-recognised in specific groups — long-term PPI users, people on diuretics, poorly controlled diabetes, alcohol dependence, and malabsorption conditions. In those situations a supplement is doing something real.
Who commonly looks for Slow Mag and why
- People with night-time leg cramps.
- People on long-term acid-suppressing medicines, where magnesium depletion is a recognised effect.
- People on diuretics for blood pressure or heart failure.
- Athletes, concerned about cramps and recovery.
- Pregnant women with leg cramps.
- People with migraine, looking at preventive options.
- People with restless legs or muscle twitching.
- People with poorly controlled diabetes.
What to check on the label before taking Slow Mag
Check the elemental magnesium, not the compound weight. As with zinc and iron, “magnesium oxide 400 mg” is not 400 mg of magnesium — the elemental content is a fraction, and it varies by salt.
Check whether the product also contains calcium, since some in this range do, and you may already be taking calcium separately.
Take it with food to reduce stomach upset, and space it from certain medicines by at least two hours — magnesium binds them:
- Antibiotics — ciprofloxacin and other quinolones, doxycycline and other tetracyclines.
- Bisphosphonates for osteoporosis.
- Thyroid medicine.
- Iron supplements.
Speak to a pharmacist or doctor if:
- You have kidney disease. This is the most important check — impaired kidneys cannot clear excess magnesium, and it can accumulate to dangerous levels.
- You have a heart block or significant heart rhythm problem.
- You have myasthenia gravis.
- You take digoxin, or a diuretic.
- You are pregnant or breastfeeding.
- Your cramps come with weakness, numbness, swelling or wasting.
- You take long-term acid-suppressing medicine and want magnesium checked properly.
Safety, interactions and when to seek professional advice
Kidney function is what determines whether magnesium is safe. In normal kidneys, excess is excreted and oral magnesium is very safe. In kidney impairment it accumulates, and high blood magnesium causes muscle weakness, low blood pressure, confusion, slowed breathing and dangerous heart rhythms. If you have any degree of kidney disease, do not take magnesium supplements without medical advice.
Diarrhoea is the commonest effect and the usual reason people stop. A slow-release form or a better-absorbed salt — citrate or glycinate rather than oxide — often solves it. Ask a pharmacist rather than abandoning it.
Nausea and stomach cramps also occur.
Long-term PPI users should have magnesium checked rather than guessed. PPI-induced hypomagnesaemia is a recognised effect that can be significant, and a blood test settles it.
Supplements do not replace assessment. Persistent cramps, twitching or weakness deserve a look — thyroid disease, peripheral arterial disease, nerve compression and medicine side effects are all treatable causes.
Seek medical attention for muscle weakness, marked drowsiness or confusion while taking magnesium, particularly with kidney disease; for palpitations or a slow heartbeat; for cramps with numbness, weakness or wasting; for severe or persistent diarrhoea; or for calf pain and swelling on one side, which needs urgent assessment for a clot.
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Frequently asked questions
What is Slow-Mag used for? Muscle cramps, magnesium deficiency, and supporting people on medicines that deplete magnesium such as long-term acid-suppressing medicines and diuretics.
Why is it “slow” release? Magnesium draws water into the bowel and commonly causes diarrhoea. Slow release spreads absorption out to reduce that.
Will it stop my night cramps? It helps if you are genuinely magnesium deficient. In people who are not, the evidence for cramps is weak — and cramps have other treatable causes worth checking.
Can I take too much? With normal kidneys, excess is excreted and it is very safe. With kidney impairment magnesium accumulates and can become dangerous — get advice first.
Why does it give me diarrhoea? That is magnesium’s usual effect. A better-absorbed salt such as citrate or glycinate, rather than oxide, is often much better tolerated.
Can I take it with my antibiotics? Separate them by at least two hours. Magnesium binds quinolone and tetracycline antibiotics and reduces their absorption.