How long does breakthrough bleeding last while taking birthcontrol pills and phentermine?

Unknown. There is some evidence that the use of both phentermine, especially phentermine in combination with topiramate, can interact with oral contraceptive pills. The side effect you describe, breakthrough bleeding, has been reported. I think it could last for many cycles based on reports. The big question is whether or not it reduces contraceptive effectiveness. I would use a back up contraceptive.
Unpredictable. BTB may last from few days to few weeks. If it doesn't subside consult your GYN.

Related Questions

How long could breakthrough bleeding last on birth control pills?

Several months. It is common to have break through bleeding in the first several months of a new birth control pill. When bleeding starts it can last several days to several weeks. If it is still occurring after 3 cycles it may be time to talk to your doctor about other pill formulations. Read more...

I'm curious as to how long is the breakthrough bleeding for ortho tri cyclon birth control pills?

Not normal. You should not have any breakthrough bleeding if it is the right one for your body.You need to go back to doctor for adjustment of your pills.Depends on what part of the cycleof your breakthrough bleeding , your doctor can change to the correct one for you. Read more...
Depends. A little breakthrough bleeding is common on the pill and only a problem if it's enough to bother you. Can occur anytime. Read more...

What do I do about ongoing breakthrough bleeding from not finishing a pack of birth control pills 2 months ago?

Consider estrogen. Breakthrough bleeding is not dangerous but can be very annoying. Typically the bleeding will stop if you take a low dose estrogen for a few weeks. If the bleeding persists then you may consider changing your method of birth control. Hope this helps. Read more...