John McKeown: Addictive cravings can destroy top competitors

When stars take drugs there is usually an addictive side to their personality that allows them to cross the line of acceptable behaviour.

There are two categories of addictive personalities that can drive them into risking their careers, reputations and fortunes.

There is the type of person – you could say the most successful sports people are like this – who has that addictive edge, that obsessive side to their personality which makes them so good. It is the same side of their personality that gets them practising, practising, practising until they are the best.

Then there are people who have both that and addiction in their heredity. They are likely to turn out to be addicted to some sort of substance, whether that be alcohol, cannabis or some other behaviour – sexually driven behaviour, for instance. They have a gene, as acknowledged by the World Health Organisation, that tends to make them addictive.

With Agassi, I think, there is perhaps the element of needing that sense of being on a high. Consciously or unconsciously, there's this need to retrieve the buzz that he gets winning on the court. Sometimes it can be that simple.

You get used to the high – but off the court, when you've nowhere else to go, you tend to reach for a substance or another person that will stimulate you in the way that you want to be stimulated.

Unhappiness has to have a role. You can see that with so many of our fallen stars when they run into trouble with alcohol or drugs. You can see it with people like Paul Gascoigne, and he describes it very well in his book about his battle with alcohol.

Taking crystal meth is an insane thing to do under any circumstances. It's crack cocaine's bad brother, a very destructive substance. There's obviously something going on in your life if you are prepared to take crystal meth.

Sport can be very regimented – often too regimented – so to take drugs or drink too much alcohol can be a way of escaping it for a time.

Equally, there can be an element of rebelliousness. Top tennis players usually start very young and in many ways they are giving up their childhood in pursuit of a dream of glory.

In these circumstances, they don't have the fun that many of their contemporaries have. They don't do the nightclubs or experiment with drugs like cocaine and ecstasy. Then, perhaps years later, they might suddenly think: "Why shouldn't I do it now?"

It's a bit of a "Sod you!", it's putting two fingers up to the world. Most of us do it as teenagers, but often these sports stars never get the chance when they're growing up as they're too busy practising their chosen sport.

It may well be that that was what Agassi was doing – taking that risk, issuing a challenge, just as most of us did when we were younger. After all, he must have known the risk of getting caught.

The author is a Harley Street addictions counsellor and psychotherapist