There are frequent questions about potency of medicines beyond expiry.
In order to illustrate the effect of exceeding expiry dates, I have calculated a series of figures showing remaining percentage potency. UK potency requirements for levothyroxine are that it remains in the range 90 to 105% from manufacture to expiry date.
This is just a simple arithmetic exercise. It does not take into account the complexity of real world medicines. (Percentages have been rounded down for display purposes.)
Assume that a medicine, when first made, is exactly at full potency.
Assume that there is a 0.25% degradation each month. This has been used as it is a low rate but still enough to show that a two year expiry date makes sense. The logic would be the same whatever the rate – just extended over a longer time if it were slower.
We have further assumed that there are steady storage conditions, and that the rate of degradation is the same every month. It is quite possible that neither of these is true. Most obviously if a package such as a sealed pot is opened or the product is removed from the controlled conditions of a warehouse to be kept on a bathroom shelf where it could be affected by humidity, heat and light. But there are many other possible reasons for this simplistic approach to fail – such as the breakdown products that accumulate affecting the rate of degradation. Or that earlier poor storage, even for a short period, could throw everything out.
After 24 months, a typical expiry period for products such as levothyroxine, it would still be within UK potency requirement at 94.17%. An extra month (from 24 to 25), one month beyond expiry date, would only drop to 93.93%.
However, you can see that three or four years make serious inroads into potency.
Month % of original potency
0 100.00%
1 99.75%
2 99.50%
3 99.25%
4 99.00%
5 98.76%
6 98.51%
7 98.26%
8 98.02%
9 97.77%
10 97.53%
11 97.28%
12 97.04%
13 96.80%
14 96.56%
15 96.31%
16 96.07%
17 95.83%
18 95.59%
19 95.36%
20 95.12%
21 94.88%
22 94.64%
23 94.41%
24 94.17%
25 93.93%
36 91.38%
48 88.68%
My summary is that we really do need to try to adhere to expiry dates. But a single extra month beyond expiry (as described) is unlikely to make a significant different in itself.
This is just an illustration. No real levothyroxine tablets will follow this exactly.