Differences between brands occur for all three injectable oxytocics and complicate a comparison between the substances. For example, one of the methylergometrines was of lower quality than the other three with initial levels of active ingredient of 75% and 38% for the two batches (see Annex 4); the same applies for certain brands of ergometrine and oxytocin. This effect has to be taken into account when comparisons are made.
Is there a difference between the stability of ergometrine and methylergometrine? First, when the mean level of active ingredient is expressed as a percentage of the initial amount, no significant difference in stability under different temperature conditions can be found between the two (see Table 3). Secondly, in two cases the stability of products from the same manufacturer can be compared at 30°C in the dark and at 2l-25°C in the light, and no difference is found (Figure 7 and Annex 6). The result of the special production is very convincing, as the products were specifically made for this purpose with exactly the same manufacturing and filling process, the only difference being in the active ingredient. The comparison between E2 and M2 from the same manufacturer is confounded by the low initial level of active ingredient of M2, but gives nevertheless the same result.
Figure 7 Comparison between stability of ergometrine and methylergometrine
Er = ergometrine; Me = methylergometrine; Da = dark, 30C; Li = light, 21-25C
We conclude that whatever difference may occur in the mean values of the different brands of ergometrine and methylergometrine is only due to the differences between products and not to the active substance per se. In other words, a certain brand of methylergometrine may be more stable than a brand of ergometrine (and vice-versa) but this is due to the formulation and/or manufacturing process and not to the difference in active ingredient.
This leaves us with the question whether the stability of oxytocin injection is any better or worse than that of ergometrine and methylergometrine. With regard to exposure to light the difference is very important, with a nearly complete loss of potency for ergometrine and methylergometrine after one year (21-27% per month), and no such effect with oxytocin. However/ also under dark storage at 4-8°C and at 30°C the average stability of oxytocin seems to be better than each of the other two, although in both cases the difference is not enough to reach statistical significance.
In conclusion, no difference in stability could be found between ergometrine and methylergometrine, other than differences between brands which are probably due to the production process. The stability of oxytocin is better than that of (methyl)ergometrine, mainly because it lacks the adverse effects of exposure to light but also because it is probably more stable when kept in the dark with or without refrigeration.