There's no entrance exam for friendship, no test for trustworthiness, and no litmus test to alert you when they become toxic. Like drinking yourself fat, it creeps up on you, Pinot by vinegary Pinot.
They start off as the perfect wingman - interesting, exciting, fun, - but after a while, the only interference they're running is with your self-esteem.
So far, so bitter.
We could, of course, finish our conversation here if the answer to the question, 'How do I make my friendships better in 2017?' was 'Avoid bitches. Drink less Pinot. The end.' But friendships are complicated beasts and most of us are hopelessly optimistic when it comes to making new ones. And rightly so. Studies have shown that good friendships mean we are less likely to suffer depression, have high blood pressure, and an unhealthy BMI. While a groundbreaking Australian research showed that strong social networks can even help us live longer.
But at a time when friendship seems more devalued than ever, when our barometer of social success is metered by social networks, should we consider a sort of friendship audit?
First we must find the rot, and it's not always so easy.
Whereas savvy modern women barely swap numbers with a date without mounting a full background check, including comprehensive social profiling, we would never dream of applying such rigour to new friends. Friends, after all, don't hurt you. Friends have your back. Then some of them stab you in it. Or else they vanish leaving you blindsided, questioning your own judgement, obsessing about your naivety, and wondering if you have become a bit of a sad sack.
Consider Gwyneth Paltrow, the consciously uncoupled doyenne of the monetised lifestyle blog.
In a 2014 post entitled 'Old Friends, True Friends, and Friendship Divorce' and widely rumoured to concern Madonna, she asked Goop readers: "What do you do when you realise that although you may have years of history, and found real value in each other in times past, that you kind of don't like a friend anymore? That, after time spent with this person, you feel drained, empty, belittled or insulted?"
This was certainly Maria's experience. The 35-year-old PR consultant from Cork found herself decoupling from a friendship after - as she puts it - a jealous outburst from a friend of over a decade.
What started out with Sheila entirely failing to acknowledge her very good friend's 30th birthday culminated in two very blunt phone calls.
"Sheila and I had been single on and off for years. We were good friends. We enjoyed the same things. We had a bond," says Maria.
'Then I met Mark and things went well, but Sheila started blanking me. After a while, Mark and I became engaged and had arranged to meet some friends for some quiet drinks to announce it.
"Then I got a phonecall.
"Sheila was ranting. She had said that I hadn't told her about the engagement, she wasn't taking a breath to listen. We had known each other 10 years but she wouldn't even hear my side of the story."
A week later, the same thing happened.
"She called again, and again she was really hostile, as much as I tried to reason with her she wouldn't listen,"says Maria. "Looking back I think she was jealous. But at the time I wondered what I'd done to make her like that.
"In the beginning it can start off with self-doubt," explains Dr Jonathan Haverkampf, psychiatrist and psychologist at Dublin's Mind And Body Works.
"The victim feels 'I'm doing something wrong' or 'I'm inferior'.
"Self-doubt and negative thoughts can cause depression and anxiety," he says.
"Competitiveness can be typical of toxic friend behaviour," Dr Haverkampf continues.
"If you feel you have to compete, or that they can't feel happy when you're successful, then they're warnings signs. Friends should want you to be happy."
But while the trite reductionist formula for a friendship break-up might be Men + Weight x Success = Trouble, sometimes as friends get older, what was once a bond becomes a bone of contention.
As it was in the case of 33-year-old Amanda and her erstwhile friend, Aoife.
"When we met in college we connected instantly. We had the same tastes in music and clothes. We knew the same offbeat comedies. And we drank a lot.
"Then college ended and I moved into a job I loved which had long, unpredictable hours, but I couldn't afford to be hungover, ever."
Aoife also got a job. It took her out of the country for weeks at a time, it also afforded her weeks off at a time, which she spent in the pub.
"I couldn't go with her but did I tell her the truth? Of course not. I stopped answering her calls or was 'busy' when she was in the country. I ghosted her before 'ghosting' was a word," says Amanda.
"In my head I blamed her for being impossible to be friends with and reduced her to nothing more than a party girl. She was away so much anyway, phasing her out was pretty easy. It took less than a year to go from daily check-ins to zero contact. These days our communications are no deeper than a Facebook like."
That was eight years ago. After ghosting Aoife, the two fell out of contact.
Indeed although the term ghosting is a phenomenon we associate with Tinder culture - in which a date is ignored by not returning calls or messages until they go away leaving the Tinderer to pursue more, promising opportunities - it seems friends have been ghosting other friends for quite some time.
Novelist Joan Brady, author of The Cinderella Reflex, is in her 50s and lives in Dublin with her husband, Dave. She says she too has been ghosted by friends, leaving her to wonder what she had done to vex them.
"It hurts like hell," says Joan, "especially when you valued the friendship. There are people who I thought would be there all my life. Maybe I have unreasonable expectations. But when someone disappears from your life you do analyse it. I go down the detective route, interviewing people and mutual acquaintances. I don't think I've ever found out what the reason was."
However, loneliness can often give pause when considering severing a friendship. Circles of friends tend to shrink over time, some marry and have children so their focus shifts to the family, some pursue opportunities outside the country, some become consumed by their career. Before long this recipe for strained and abandoned friendships leaves the survivors wary losing any more of their number.
As Maria says, "I moved city which meant I lost a lot of friends. And the older you get, your friends start to marry off, have families and move away. You let your standards slip. You end up going out with people you wouldn't have before, because you don't have a good support network around you. And if they become toxic, that puts you in an even more vulnerable position."
New people bring new challenges and Irish author Ally Bunbury's debut novel concerns a similar subject. The Inheritance follows an Irish girl, Anna, who moves to London and falls in love with a charming Scottish heir and art dealer, and in so doing incurs the wrath of her beautiful American actress 'friend', Sofia.
Ally, who is 40 and lives in Co Carlow with her husband, Turtle, and two daughters, agrees that she is more selective with new friends.
"Older friends are part of your history, they're part of you. With new friends you're more cautious because there's only so much time. I am probably more choosy as I get older. You're less likely to bear someone who isn't a similar temperament."
Ally says she has kept friends from childhood and her early 20s but admits that "keeping friends is maintenance". "My friends live in different countries, different counties. So you have to make a proper conscious effort, spend some quality time and make sure they're okay. And hope they'll do the same for you."
And we all, whether we care to admit it or not, need strong friendships. Support networks are essential to reinforce our psychological scaffold. A 2009 study from published in the Journal of the National Medical Association found that those with less social support were most likely to suffer anxiety and depression.
This concern is all the more pressing when one considers the increasing number of women who are single into their 40s, 50s and beyond.
Dr Haverkampf encourages clients to get out there.
"I encourage them to pursue their interests and remain socially active. If people have been alone for a long time, finding new friends can be difficult so you need to explore new social spheres," he says.
This also applies to the young, who, despite a facility with social media, can struggle to make friends offline.
"Some people in their 20s may need to learn social skills. It might not be apparent but many younger people feel pretty alone. Withdrawing from friends and loneliness can be a sign of depression. If someone feels lonely or depressed or not very social then I would suggest they talk to a counsellor," he adds.
With all of this, it's no surprise that it can be daunting to weed bad friendships out of your life, even if it's better in the long run.
It's rather germane then that comedian Eleanor Tiernan describes toxic friends as being like "wisdom teeth". "They start to become painful when you're in your 30s. The roots are usually very deep and they leave a bit of a hole after they've gone."
So it proved for Amanda.
"I miss her deeply. The older I get the more I realise I missed out on something special. I thought I was being grown up by phasing her out but I was just judging her and letting her down. And I lost something precious because of it."
Donna Breen, counsellor with Spectrum Health, explains:
1 Ignoring boundaries
"If your friend is pushing you to keep meeting or going out when you've made it clear that you can't, they're starting to force your boundaries. Or if someone demands that you listen to their problems but never reciprocates, then there is an issue."
2 Not acknowledging your requests for help
"Sometimes friends fall out because grievances go unsaid. You might need to ask for help, or stand your ground when it comes to boundary-setting. If these requests are ignored then this is another warning sign."
3 Caustic comments
"Poisonous behaviour or nasty comments are a hallmark of toxic friendship. This can be when you're on your own or in front of friends or colleagues. If you feel the friendship is worth saving, then address it in a relaxed, level-headed manner and certainly in a sober environment. Give them the opportunity to accept the behaviour. If they refuse then you have a decision to make."
4 Time to step away?
"If addressing the situation has not been successful then it's probably time to step away. In these situations, having space and time to assess is important. It's not about falling out with people but rather giving yourselves time to figure out if the friendship is worth it."
THE DUNBAR NUMBER
Dunbar's Number is a guideline for the number of stable social relationships the average individual can sustain. First proposed in the 1990s by psychologist Robin Dunbar at University College London, it actually consists of a series of numbers corresponding to the intimacy of the friendships involved.
Casual friends (you'd invite to a large party)150
Close friends (you'd invite to dinner)50
Intimates (you'd turn to for support)15
Inner circle (family and very best friends)5