Introduction Patients with a high output ileostomy often have hypomagnesaemia. Oral magnesium therapy may be unsuccessful. Topical magnesium therapy may offer a novel mode of replacement. This phase II clinical study using BetterYou magnesium oil spray primarily aimed to determine if the spray will maintain or increase serum magnesium in these patients.

Methods Outpatients with an ileostomy formed more than 6 months prior to inclusion and having chronic hypomagnesaemia (serum magnesium level less than <0.66 mmol/L) at enrolment and either:

(a) a further outpatient serum magnesium (Mg⁺²) level <0.66 mmol/L in the last 3 months, or

(b) needing regular intravenous magnesium infusions at least once every 6 weeks for more than 18 weeks, were recruited.

Exclusion criteria included severe hypomagnesaemia (<0.25 mmol/L), diuretic use, and medication alterations (including supplements and magnesium infusions) within 4 weeks of enrolment.

Recruits applied 10 sprays twice daily for 6 weeks, delivering a topical elemental magnesium dose of 150 mg/day. Serum and whole cell Mg⁺² levels were measured at enrolment, and at weeks 1, 3 and 6. Vitamin D levels were measured at enrolment; 24 hour urinary magnesium levels were measured at enrolment and week 6.

Treatment response is defined as a serum Mg⁺² level rise >0.10 mmol/L at week 6, or the avoidance of a planned magnesium infusion during the trial without a fall in serum Mg⁺². Patients with serum Mg⁺² <0.25 mmol/L or requiring additional magnesium supplementation during the study were withdrawn.

Results 7 patients entered the study. 6 patients completed it; 1 was withdrawn at week 4 due to hospitalisation for Crohn’s disease. All 6 patients had normal enrolment vitamin D levels (>45 nmol/L), and 5 of 6 had 24 hour urinary magnesium levels below the minimum detection limit (<0.28 mmol/L).

1 of the 6 patients avoided their planned six-weekly magnesium infusion, maintaining a stable serum Mg⁺² level (0.59 mmol/L at enrolment, 0.62 mmol/L at week 6) and increased 24-urinary magnesium output. 2 of the remaining 5 patients were treatment successes, with a serum Mg⁺² rise of 0.27 and 0.13 respectively. No patient had a fall in serum Mg⁺² of greater than 0.07 mmol/L between enrolment and week 6.

Serum and whole cell Mg⁺² correlation is strong and near linear (Pearson’s r=0.92, p<0.01). All 6 patients complained of muscle cramping at enrolment; 5 reported significant improvement or complete resolution of cramping by week 3.

Conclusions 3 of 6 patients were treated successfully with BetterYou magnesium oil spray. Serum magnesium is a reliable surrogate for whole cell magnesium.