Many of our closest pals have been by our side for decades, through life's ups and downs.
That's why it can be particularly bewildering and upsetting to realise that a treasured friendship has somehow soured and that your friend's behaviour towards you has changed for the worse. Maybe forever.
We've all experienced a period when a once-close friendship started to show cracks, but how do you know whether it's really worth salvaging?
Life coach Gemma McCrae believes the best way to answer this question is to describe what a friend actually is.
"A friend is someone you trust, who makes you feel good, supports you and wants the best for you. A toxic friend is the opposite," she says.Relationship expert Ar'nie Krogh says toxic friends are mostly takers and rarely givers: "The only thing they give out is the ability to make you doubt yourself and your choices. If you're experiencing negativity, it's time to do something about it."
Holistic therapist David James Lees says on a day-to-day level you may notice phone calls not being returned or get-togethers being avoided.
"You may also notice your friend becoming dismissive, critical or judgemental in a way that hasn't happened before," he says.
Ar'nie reveals recognising a toxic friend isn't as easy as you think. "Some toxic behaviours are so subtle that we question ourselves for even considering them," she says.
If you feel like you're walking on eggshells because you never know when your friend's mood might change, or the conversation seems to always revolve around their problems and their life, and you feel like they don't take your problems seriously, there's a good chance things are going sour.
What's more, if you start to heave a sigh of relief when they leave, that's when you know it's a toxic friendship.
"Perhaps their circumstances have altered for the worse. For example, they may have become widowed and might be jealous of your relationship or they could be envious of your retirement while they're still working," she says.
Although a single major disagreement between friends can cause a rift, David says toxic friendships are more likely to develop slowly over months or years.
"Don't ignore early warning signs of things turning sour – address them as soon as possible by talking to your friend," he advises.
Instead of waiting for them to make the first move, be proactive to deal with any avoidable misunderstandings before things go too far and you both end up miserable.
This is the question you have to ask yourself but it can be difficult to answer on your own. If you're unsure whether you might be overreacting, try talking to another friend or family member about the situation to get an outsider's viewpoint on your friend's behaviour.
David says the best way to judge whether a change in your friend's behaviour is a blip or something more serious is to consider what's happening in their life, or even in your life, which may be causing either of you to behave differently.
"Address your concerns by talking to your friend. Firstly, acknowledge how much their friendship means to you and then ask some questions, such as, 'Have you noticed a change in our relationship?' and 'What can we do about this?'"
Sometimes, despite your best efforts, there's nothing you can do to save your friendship – you have to be brave and simply walk away.
"When people are determined to be negative, no amount of persuasion can convince them to change," Ar'nie says. Instead, they have to want to change their behaviour and recognise how it's affecting your relationship.
"If you find the friendship is beyond repair, you should deal with the situation with truth and honesty," David advises.
Explain that you feel your friendship isn't helping either of you and why you think it'd be best if you both moved on. Sometimes staying away, no matter how painful it is, can be good for them, too.
David says,"Wish your old friend well, acknowledge how grateful you are for the good times you've shared and celebrate your joint adventures. Then you can move on, knowing you've done your best for your friend and refocus on your future."