Harold asked
I have eaten 1/2 grapefruit for 20+ years with breakfast. Now on Flomax (Tamsulosin) 0.4 mg. Is there a possible interaction with grapefrui?. Can I alternate Flomax to every other day and still reap the benefit of this medication along with 1/2 grapefruit with my breakfast daily?
Answer
It is generally not recommended to combine grapefruit (the solid food or the juice) with Flomax (tamsulosin). If they are consumed together, it should be done with caution and you need to be aware of the potential increase in side effects.
Grapefruit Drug Interaction Mechanism
Grapefruit can interact with medications via several mechanisms. Most notably it can:
- Inhibit the metabolizing enzyme CYP3A in the small bowel wall, potentially incresasing medications to unsafe levels.
- Inhibit the drug transporting enzyme OATP (organic anion transporting peptide), potentially reducing absorption of drugs.
The primary mechanism by which grapefruit interacts with Flomax is via inhibition of the CYP3A metabolizing enzyme. The inhibition of this enzyme can potentially increase levels of Flomax to higher than usual levels, putting you more at risk for the following side effects:
- Excess sedation and dizziness
- Overall lack of energy
- Nausea
- Runny nose/Cold like symptoms
Separating Flomax administration and consumption of grapefruit typicality does not assist in avoiding this interaction. Upon grapefruit exposure, studies shown that about half of the enzyme activity will be recovered in about 24 hours, but full recovery can take much longer, up to 3 days!
There have been no clinical studies specifically testing how grapefruit juice affects Flomax levels, and the interaction is simply theoretically. Nevertheless, Flomax should be used with caution.
Flomax is considered to be a fairly high therapeutic index drug, meaning that it would take very large doses to become toxic. You may be OK and relatively unaffected by the interaction, but it is important to know about and to watch for side effects. Especially since you are in an established routine in regard to grapefruit consumption, you are your doctor may decide to try using the medication and see how you react.
