There is some controversy as to whether or not Diflucan (fluconazole) will affect birth control to the point where it is not effective in preventing pregnancy. The majority of the data available shows that fluconzole actually INCREASES the estrogen and progestin contained within birth control. This means that theoretically, taking fluconazole and Junel should not decrease the efficacy of the medication.
Fluconazole is a known CYP3A4 (Cytochrome P450 3A4) inhibitor. CYP3A4 is a metabolizing enzyme mainly found in the liver and intestines and is the predominant enzyme that metabolizes, or breaks down the estrogen component of birth control, ethinyl estradiol. When the enzyme is inhibited (meaning it is no longer working at full strength), the concentrations of ethinyl estradiol in the body are actually increased over what they would normally would be if the enzyme wasn't inhibited. Many progestins as well follow this mechanism of metabolism.
The Prescribing Information, or Package Insert, for fluconazole shows empirical data on how it affects the hormones contained in oral contraceptives. It references 3 studies in which ethinyl estradiol and progestin concentrations were measured after giving a dose of fluconazole to a patient.
- In the first study, fluconazole was given at a dose of 50mg per day for 10 days. Both the estrogen and progestin concentrations were raised on average by 6% and 17% respectively.
- In the second study, patients were given a 200mg dose of fluconazole. Both the estrogen and progestin concentrations were raised on average by 38% and 25% respectively.
- Lastly, in the third study, patients were given a once weekly dose of 3o0mg of fluconazole. Both the estrogen and progestin concentrations were raised on average by 24% and 13% respectively.
While the above data shows an average increase in birth control hormone concentrations after fluconazole dosing, the studies did show that a small number of individuals showed a slight decrease in concentration. While study 3, referenced above, showed an average increase in concentrations, there were 3 people in that study that had a drop in levels. While uncommon, this is very concerning as we want to take the most prudent and cautious approach available to prevent an unplanned pregnancy. Moreover, data from systemic reviews shows that a very small number of patients can have decreases up to 47% of ethinyl estradiol concentrations. Putting all this together, the available data seems indicate that decreases are uncommon and likely due to random variation.
So this puts us in a precarious position: We can most likely expect birth control levels not to be negatively affected by fluconazole dosing but there have documented cases in which there are problems. With this information in hand, it is prudent to recommend back up birth control while taking fluconazole to mitigate the very low chance that birth control hormone levels are decreased as a cautious approach.
You mention in your question that you did use backup birth control but it failed. It would be a personal decision as to whether or not you want to use Plan B. The data shows that the vast majority of people have an increase in birth control levels while on fluconazole so there is a good chance you aren't at risk for pregnancy. However, since we do know that a small number of people are affected negatively by fluconazole dosing, if you want to use Plan B as an extra safety measure, that is certainly understandable and a prudent approach for many.