I wouldn't risk it. The cup is easy to move around, and the sex action could cause the suction to break and let sperm through. The spermicide would sit low in the cup, and the sperm itself could manage to get at the cervix without ever coming into contact with it.

my guess is there is info about that on the website for whatever cup you have. you're probably not the first person to think of it.. there's probably a faq on it. but my guess would be that it'll say don't do it.

I think, along with the other mentions, that you would need to be concerned about the spermicide breaking down and destroying your cup. The cup isn't meant as a birth control.

She's speaking of a stem so it must be a re-useable.
But having sex with a re-useable cup in is not unthinkable. I've never tried it, but I've read several times about it here.

i'm going to have to disagree with what has been said.

if you have the room for your cervix to be in the cup during intercourse, then i'd have to say that i would work. spermicide would not hurt the cup as most of the cups are medical grade silicone. i have a femcap (cervical cap) and it is also made of medical grade silicone and it is meant to be used along with spermicide.

having said that, you should be aware that cervical caps + spermicide (or cup+spermicide, in your case) range from being about 80-92% effective. so either use a backup or be prepared for parenthood. also, some women have reactions to spermicide. although this hasn't been fully tested, you CAN make a homemade spermicide using 100% aloe vera gel plus PURE lemon juice (real stuff from real lemons). use it in about a 1 tbsp aloe + 5 drops of lemon juice ration.

You get much better protection from a condom.

With perfect use, yes.
But with typical use condoms and cervical caps have pretty much the same failure rate:
condoms = 15%
cervical cap = 16%

See this chart for a comparison of contraceptive effectiveness.

They aren't designed to be used during sex - I imagine it would likely break the seal.

Everybody's body is different.
For me, my cup fits pretty much like a cervical cap - I used one for a few years back in the mid 90's and never got pregnant with it...
I actually have had PIV sex unintentionally and then intentionally with the cup in on a few occasions and, in my experience, as long as I have a good seal on my cup to begin with, sex does not break the seal.
That said, I certainly don't use my cup as contraception - that's what I have an IUD for. ;)

a tampon in spermicide

That just gives me the heebejeebes on so many levels!!!

I'm certainly not advocating using a cup for contraceptive purposes.

if someone doesn't have a huge cirvex and the small loona cups fit so snug and you can get them with out a stem is this much different to using a femecap. at the moment I'm not bothered as to when i get pregnant, later would give more planning time but due to health issues i may not have that long with my ovories so i ask this as someone who's not interested in a perminant or hormonal style of bc.

your taking about using your cup as a diaphragm and I wouldn't rec doing that. Diaphragm's are usually fitted by health care professionals where as menstural cups aren't. Fitted and inserted correctly diaphragms have a 94% but stress thats when fitted correctly and inserted correctly. There's a much bigger chance of your cup moving so the % for effectiveness as birth control would be a hell of alot lower.

Why risk it? Why not just get fitted for a diaphragm?

she's talking about using her cup as a cervical cap, not diaphragm

Argument remains the same - both cervical caps and diaphragms should be fitted by a health care professional in order to get the highest protection from them, the risk of pregnancy by using a cup would be high as it's not designed for that function, why take the risk when it's far easier to just get a proper cap or diaphragm? The cap is held in place by the cervix so needs to be fitted correctly to make sure it doesn't move during sex, cups aren't designed to be that fitted.

Exactly.

If that actually worked, I'm sure they would have started marketing cups as dual function menstrual hygiene + birth control by now. That is not their intended purpose. Unless you want to get pregnant, I would not do it.

While I wouldn't recommend using a menstrual cup as contraception, there actually is a diaphragm under development that may be marketed for menstrual protection as well.
I actually posted about it a while back here.

Basically, because cervical barriers are thought to provide some protection against STI transmission for women by covering the vulnerable cervical cells, there actually is a diaphragm undergoing feasibility testing for contraception, STI protection, and menstrual hygiene. The menstrual hygiene aspect is being featured as a bonus/selling point to encourage women at risk to use the method to protect themselves if/when their partners refuse to wear condoms... prevent unwanted babies, STIs, and stains in your panties - all in one discrete diaphragm!

Since cups are designed to sit lower in the vagina, I'd say no. Plus... um, what about the holes? ;)