The mean levels of active ingredient, stated in the left half of Annex 4, are distorted by the fact that three of the eight batches were already far below USP/BF standards at the start of the study, with values of 38-82% of the stated content. To overcome this problem the level of active ingredient over time has been expressed as a percentage of the initial content (right half of Annex 4).
When stored for one year at 4-8°C in the dark, the mean loss of potency of methylergometrine is 4% of the initial content (95% CL 0-8%, range 0-13%). At 30°C in the dark it is 18% (95% CL 6-31%, range 2-45%) and at 21-25°C in the light the loss is over 90%. Two weeks at 40°C resulted in a mean loss of 3% (range 0-11%). As with ergometrine, there are considerable differences in stability between the four brands, as already illustrated by the presence of one defective product. However, at 21-25°C in the dark the other three show a similar pattern of stability (see Annex 5/2).
In the only field study that included methylergometrine, the WHO/UNICEF transport study7,8, the drug lost 2% of its active ingredient during 8 weeks of transport from Copenhagen to Lagos and Entebbe. This study included only one brand and the result is in line with that of the same brand in the present study.
The results of the simulation study suggest that methylergometrine may lose, on average, about 4% potency (range 0-13%) per year when stored at 4-8°C in the dark, and about 18% (range 2-45%) at 30°C. The influence of light at room temperature results in a rapid loss of active ingredient. Some products are more stable than others and the average value should be interpreted with caution.