How to Apply SU Mag: A Guide to Using Magnesium Sulfate Paste

October 6, 2025 •

4 min read

Magnesium sulfate paste, often called a "drawing ointment," is a traditional remedy used for various minor skin complaints, including boils and splinters [1.3.6]. This guide explains how to apply SU mag paste for effective results.

Quick Summary

Learn the proper technique for applying magnesium sulfate paste. Stir the paste, apply a liberal layer to the clean, affected area, and cover with a sterile dressing. Reapply once or twice daily.

Key Points

What it is: 'SU Mag' refers to Magnesium Sulfate Paste, a drawing ointment for skin issues like boils and splinters [1.3.6].
How it Works: It operates via osmosis, drawing moisture and pus from under the skin to the surface [1.7.1].
Application Steps: Stir the paste, apply a thick layer to the clean, affected area, and cover with a sterile dressing [1.2.1, 1.2.4].
Frequency: Change the dressing and reapply the paste once or twice daily until the issue resolves [1.4.4].
Primary Uses: It is mainly used for treating boils, carbuncles, and for helping to remove small, embedded splinters [1.3.2].
Safety First: Do not use on open wounds (other than a boil) or if you have an allergy to the ingredients [1.3.5, 1.3.7].
When to See a Doctor: Consult a physician if the condition worsens, you develop a fever, or there's no improvement after a few days [1.2.4, 1.7.6].

What is 'SU Mag' or Magnesium Sulfate Paste?

'SU Mag' is likely a colloquial term for Magnesium Sulfate Paste, a widely used over-the-counter medication known as a "drawing ointment" [1.3.6, 1.3.7]. Its primary active ingredient is dried magnesium sulfate (commonly known as Epsom salt), mixed with glycerol and a small amount of phenol [1.2.3, 1.3.1]. Magnesium sulfate has been in medical use since as early as 1618 [1.3.2].

This paste works through a process called osmosis [1.7.1, 1.7.4]. The dried magnesium sulfate is hygroscopic, meaning it attracts water. When applied to the skin over a boil or splinter, it creates an osmotic gradient, drawing moisture, pus, and other fluids out from the inflamed tissue and towards the paste [1.3.2, 1.7.1]. The glycerol helps keep the paste moist, sustaining this drawing action over several hours [1.7.5]. This process helps to bring boils to a head, draw out foreign objects like splinters, and reduce painful inflammation [1.7.4, 1.8.4].

Primary Uses of Magnesium Sulfate Paste

This ointment is a staple in many first-aid kits due to its versatility [1.3.6]. Its primary applications include:

Boils and Carbuncles: It is highly effective at drawing out the pus from these painful, bacteria-filled skin abscesses, reducing pressure and promoting natural drainage [1.3.2, 1.7.2].
Splinters and Foreign Bodies: The paste can help draw small, embedded objects like wooden splinters, thorns, or glass shards closer to the skin's surface, making them easier to remove with tweezers [1.3.2, 1.8.3, 1.8.5].
Minor Skin Infections and Abscesses: It aids in managing small, localized skin infections by helping to clear out pus and debris [1.3.6, 1.7.2].
Inflamed Ingrown Hairs: By drawing out pus, the paste can help reduce inflammation and bring the trapped hair to the surface [1.3.6].

How to Apply SU Mag (Magnesium Sulfate Paste): A Step-by-Step Guide

Proper application is key to maximizing the effectiveness of magnesium sulfate paste. Always wash your hands before and after use [1.2.4].

1. Preparation

Gather your supplies: You will need the tub of magnesium sulfate paste, a clean applicator (like a sterile tongue depressor or the handle of a teaspoon), and a clean, sterile dressing or plaster large enough to cover the affected area [1.2.1, 1.8.1].

2. Clean the Area

Wash the affected skin: Gently clean the area around the boil or splinter with mild soap and warm water, then pat it dry [1.2.6]. Ensure there is no broken skin, unless it is a minor localized infection like a boil [1.2.1, 1.3.5].

3. Stir and Apply the Paste

Stir the paste well: The ingredients can separate over time, so it's important to stir it until you have an even, smooth consistency [1.2.3, 1.2.4]. Using your clean applicator, scoop out a generous amount of the paste.

Apply liberally: Spread a thick layer of the paste—about 5mm thick—directly onto the affected area. Do not rub it in [1.2.1, 1.4.6]. Ensure the entire boil or the skin over the splinter is completely covered [1.7.2].

4. Cover with a Dressing

Secure a dressing: Place a sterile, non-adhesive dressing or a plaster over the paste [1.2.1, 1.8.3]. This keeps the paste in place, prevents it from rubbing off on clothing, and protects the area from dirt and bacteria [1.7.2].

5. Frequency and Duration

Reapply as needed: The dressing should be changed and the paste reapplied once or twice a day (e.g., every 12-24 hours) [1.4.4, 1.4.6]. Gently wash off the old paste with warm water before each new application. Continue this process for a few days until the boil drains or the splinter becomes accessible. If there is no improvement after two to three days, you should consult a pharmacist or doctor [1.4.3, 1.8.1].

Comparison of Topical Boil Treatments


Treatment	Mechanism of Action	Best For	Key Considerations
Magnesium Sulfate Paste	Osmotic action; draws out fluid and pus from under the skin [1.7.1].	Unbroken boils, carbuncles, splinters, and minor abscesses [1.3.2].	Should not be used on large areas of broken skin [1.2.4]. Very effective for bringing infections to a head.
Ichthammol Ointment	Also a drawing salve, thought to have mild antiseptic properties.	Similar uses to magnesium sulfate, including boils and splinters [1.3.6].	Has a very strong, distinct odor. Evidence for its drawing action is less defined than the osmotic effect of magnesium sulfate [1.5.2].
Warm Compresses	Moist heat increases blood flow to the area, encouraging natural drainage.	Early-stage boils to help them mature and come to a head [1.3.6].	Requires frequent application (multiple times a day for 10-15 minutes). It's a non-medicinal first step.
Topical Antibiotics	Kills bacteria on the skin's surface to prevent or treat infection.	Minor cuts and scrapes after a boil has drained to prevent secondary infection.	Does not have a "drawing" action to bring pus to the surface.

Precautions and When to See a Doctor

While generally safe for most people, there are important precautions [1.3.6].

Do not use on broken skin or large open wounds, unless it's a minor infection like a boil [1.3.5, 1.3.7].
Do not use if you are allergic to magnesium sulfate or any other ingredients [1.3.2].
Avoid prolonged use without consulting a doctor [1.3.5].
Side effects are rare for topical use but can include skin irritation. If this occurs, stop use and consult a pharmacist [1.2.1, 1.6.2].

You should seek medical advice if:

The boil is very large, painful, or located on your face or spine.
You develop a fever or feel generally unwell [1.7.6].
The condition does not improve after several days of treatment, or it worsens [1.2.4].
The area shows signs of a spreading infection, such as significant redness, warmth, or red streaks [1.2.1].
You have a weakened immune system or diabetes.

Conclusion

Magnesium sulfate paste, or 'SU Mag', is an effective, time-tested remedy for drawing out minor skin infections like boils and removing splinters [1.3.6]. By following the correct application procedure—cleaning, applying a thick layer, and covering the area—you can leverage its powerful osmotic action to find relief. However, it is crucial to recognize its limitations and consult a healthcare professional for more severe or persistent conditions.

For more information on medication safety, you can visit the U.S. Food and Drug Administration.

Frequently Asked Questions

SU Mag, or Magnesium Sulfate Paste, is primarily used as a 'drawing ointment' to treat boils, carbuncles, and to help draw out small splinters or thorns from under the skin [1.3.2, 1.3.6].

The paste works through osmosis. The high concentration of dried magnesium sulfate draws moisture, fluid, and pus from the inflamed area into the dressing, which helps bring the infection to a head [1.7.1, 1.7.4].

You should typically apply the paste once or twice a day. Clean the area, apply a fresh, thick layer of paste, and cover it with a new, clean dressing each time [1.2.4, 1.4.4].

No, you should not use it on open or broken skin. It is intended for use on intact skin over a boil or splinter to draw out impurities from underneath [1.3.5, 1.3.7].

It typically starts working within hours, but it may take one to two days to bring a boil to a head so it can drain. If there's no improvement after a few days, you should see a doctor [1.2.4, 1.4.4].

It is generally recommended to use it with caution on the face and avoid the eye and mouth area. For facial boils, it's best to consult a pharmacist or doctor before use [1.2.4].

Both contain magnesium sulfate, but the paste is a concentrated formula with glycerol designed for topical application as a drawing agent. Epsom salts are crystals intended to be dissolved in water for soaking [1.4.4, 1.7.2].