Warning: it might make your friend feel better, but what do you do if being her confidante is giving you the sleepless nights?

My friend - let's call her Samantha - confessed that she was on the verge of having an affair. Now for my confession: I didn't slap her hand and tell her to behave herself. Instead, I leaned forward across the restaurant table, practically drooling. Who was he? Had they, you know, done it? Did her partner know what she was up to?

During the weeks that followed, I wished I'd never asked. I was deluged with every intimate, icky detail. One day she called me at work, babbling, 'I'm out with you tonight, OK?'. Without applying for the job, I had become Samantha's alibi. It wasn't a role I relished.

One evening, at a party, Samantha's partner (who I liked very much) cornered me and said, 'I was in that bar at the end of your road last night. I popped round to see you...' My heart started thumping. I was supposed to be out with Samantha (while she had dinner with her lover). But did her partner know that? 'I was out,' was all I could muster as I turned as hot and glossy as a tomato.

A friend's affair is a tense, nervy business. When you're involved, unwittingly or otherwise, you are no longer just a friend. You are a round-the-clock counsellor. Her needs change with the wind: one minute she wants reassurance that a fling 'isn't so terrible'. Next time you see her, she requires a shoulder to cry on and a steady supply of red wine.

Friends like these can be a major source of anxiety. 'A good, wide circle of friends is a great buffer against stress,' advises Professor Stephen Palmer, Director at the Centre for Stress Management. 'But possessive, demanding friends aren't. You might try to be the perfect friend and tell yourself, "I've got to help, I must see her," but this only makes you feel guilty.'

Hang on. Who should be feeling guilty here? If your friend's extra-curricular sex life is giving you a headache, here are six good reasons to butt out:

1. You don't know the whole storyA friend with a volatile love life may only give you edited highlights. Armed with snippets and half-truths, you are simply not equipped to offer worthwhile advice. Steer her in the direction of a counsellor or therapist instead.
   2. Getting involved wrecks your friendship. What starts as confessional sessions over drinks can escalate out of control. Lucie, 34, had no qualms about helping out her best friend, Helen: 'Just after Christmas, she arrived, all tearful, with her clothes stuffed into two carrier bags. Her marriage to Nick - also a great friend of mine - was over.' It soon transpired that Helen had omitted to mention that she had been having an affair with a work colleague, the real reason for leaving her husband. 'That changed everything. I was furious with Helen. I couldn't have her living here, seeing this other guy. So I had to ask her to leave.' Not surprisingly, their friendship hasn't recovered.

   3. Her affair can sour your own relationship. When my friend Samantha's affair was at its most complex, my partner became moody and distant. Quite reasonably, he felt threatened by two women, constantly huddled together, discussing extra-marital affairs. Your friend's love life can even make you question your own relationship: is it really so perfect? And if she can get away with an affair, could you?

   4. You don't have time to play counsellor. Being an unofficial therapist gobbles up enormous quantities of time and energy. Perpetually cancelling drinks with your friend is the coward's way out: 'Saying, "We must meet up" - and having no intention to do so - is counter productive,' says Professor Palmer. 'Make a firm arrangement, or forget it altogether. It feels more empowering than all that guilt.'

   5. No one needs 3 a.m. phone calls. Your friend is getting double the sex; all you're getting is a red-hot ear from lengthy phone conversations - and sleep deprivation.

   6. You are not her only friend. Don't listen to protests of, 'You're the only person I can talk to about this.' Would you load so much responsibility onto someone you cared about? Ask her to stop involving you. Never step in as an alibi (if you let something slip she'll blame you and forget all the times you've saved her bacon). If necessary, get firm and tell her that knowing every intimate detail is like being involved in a menage a trois - without the sex. And remind her that this is her affair. Not yours.

   Do you have a difficult friend? Have you ever been involved in a friend's affair? Talk about your relationships with like-minded iVillagers.