Magnesium will not save you. The cramp at kilometre 32 is not a deficiency. It is a sentence about how you trained. The cure is in the eleven weeks before the race, not in the bottle on your kitchen counter.
This is the part the supplement aisle does not want you to read. So let's read it.
The story you have been sold
You cramped at the Hyderabad Marathon last December. Some senior runner at the WhatsApp group told you it was magnesium. You bought 250mg tablets at the pharmacy on Banjara Hills. ₹450. You took them religiously through your next block. You cramped again at kilometre 28 in Mumbai.
What went wrong was not the magnesium. What went wrong was the diagnosis.
What the actual research says
The cleanest review on this is in the Cochrane database. The conclusion is uncomfortable. Magnesium supplementation has not been shown to prevent exercise-associated muscle cramping in otherwise healthy adults. Not in trained athletes. Not in recreational runners. Not in older adults. The evidence is thin and the effect, when present, is small.
That is not a hot take. That is the body of literature, summarised by people whose job is to summarise it without favour.
Magnesium has real uses. Pregnancy cramps. Diabetic complications. Documented deficiency. None of those is you, on kilometre 32 of a marathon you didn't train enough for.
What is actually causing your cramps
The evidence points somewhere else entirely. Cramps in distance runners are most strongly associated with neuromuscular fatigue. You are running faster than you trained to run. You are running longer than you trained to run. Your motor neurons are firing in patterns they have not rehearsed.
The muscle fires. It does not turn off. It locks.
That is the cramp.
The pattern is honest if you look
Look at when you cramped. Was it your first marathon? Yes. Was your weekly mileage in the block under 50 kilometres? Yes. Did you go out faster than your long-run pace? Yes. Did you skip the last two long runs because of work? Yes.
That is your cramp. The bottle has nothing to do with it.
Cramps are the body's way of refusing to do work you didn't prepare it for.
The other suspect — sodium
If we are being honest about electrolytes, sodium has stronger evidence than magnesium. Sweat sodium losses in heat — and Indian race conditions in Mumbai, Hyderabad, Chennai, Delhi, Bengaluru are heat conditions for most of the year — are real and meaningful.
A 2019 review in Sports Medicine on exercise-associated muscle cramping concluded that fluid and electrolyte imbalance, particularly sodium, may contribute in some endurance runners, though the dominant factor remains neuromuscular fatigue.
So if you are going to spend money on something, the bottle to look at is not magnesium. It is salt. Specifically: how much sodium you lose in an hour of hard running in Indian summer heat, and how you are replacing it.
The cheap version of this experiment
Drink your usual hydration mix in a 90-minute run at marathon pace. Taste your sweat at the end. Lick your forearm. If the salt taste is intense, you are a salty sweater. Get an electrolyte mix with 700-1000mg of sodium per litre. Practice it in training. Use it in the race.
This costs about ₹300 a month. It will do more for your cramp problem than ₹450 of magnesium has done in the last decade.
The thing nobody wants to admit
Most runners who cramp at the end of a marathon are runners who trained for a 30-kilometre race and tried to run 42.
You cannot supplement your way past undertraining. You cannot pill-stack past a 20-kilometre long run when the race is 42. You cannot drink your way to readiness for a load you did not build.
The cramp is a teacher. The cure is the training block you do next.
What actually prevents marathon cramps
Long runs. Lots of them. At least three runs in the 28 to 35-kilometre range in the block. Weekly mileage that exceeds 60 kilometres in the peak weeks for a first-time marathoner aiming to finish strong.
Pacing. Going out at your trained long-run pace, not the pace the start-line adrenaline wants.
Strength. Two sessions a week of compound lower-body lifting. The exercises library has the work.
Heat acclimation. If your race is hot, train hot. Run in the afternoon in the last block, not just in the cool dawn.
Electrolytes. Real salt, in real amounts, practised in training.
That is the list. That is what works. That is what no advertisement will sell you because there is nothing in it to put on a shelf.
So should you take magnesium at all?
If you have a documented deficiency, treated by a doctor — yes.
If you sleep badly and want to try magnesium glycinate before bed for sleep quality — fine, the evidence is mixed but mild. The downside is small at ordinary doses.
If you are taking it because your friend in the running group told you it stopped his cramps — stop. He stopped cramping because he trained more, not because of the pill. Survivorship bias has put more magnesium on Indian kitchen counters than the Cochrane review will ever pull off.
The truth: most marathon cramps are training cramps wearing a supplement disguise.
What to do instead
Build the block. Run the long runs. Lift twice a week. Practice race-pace fuelling. Drink real electrolytes in training, not race day. Sleep more than you are sleeping now. See the recovery guide for the rest of it.
If something hurts in a way that is not ordinary soreness, the injuries reference covers the difference. If you want a training block that actually prepares your legs for 42 kilometres instead of guessing, the STRIDD plan generator builds one. The calculators can set the right paces. For the broader frame, see the Running Lab reference.
The cramp is information. Read it correctly.
Your next marathon is not waiting in a bottle. It is waiting in eleven Sundays of long runs you have not run yet.