Few track and field generations have produced an individual quite as successful as Mo Farah.

He is, in every corner of the globe, considered one of the greatest athletes of all time. Four Olympic gold medals, twice doing the double-double, six world titles, five European titles plus a raft of British and European records is something only a few could even dream of accomplishing.

In particular, his first gold medal of the 2012 Olympics, when he won the 10,000m in front of 80,000 jubilant fans in London’s Olympic Stadium on Super Saturday, will live long in the memory.

It’s why the end of his career – he will race today in London in the Big Half before bowing out next Sunday at the Great North Run – marks the end of an era.

Farah will, almost certainly, never see his achievements surpassed by a fellow Briton, not in his lifetime anyway.

But Farah’s story was remarkable for more than just what he achieved on the track and road. The 40-year-old was born in Somalia and came to the UK as a child; he has said he was trafficked to this country at the age of nine before being forced into child labour. 

It was a start few world-class athletes have endured and so what he has gone on to achieve is astonishing.

Yet Farah is not considered by everyone to be the all-encompassing hero. He is viewed by many, myself included, in a very different light to the likes of Jessica Ennis-Hill, one of Farah’s peers who navigated her career free of any controversies.

Farah, in contrast, was rarely far from a scandal, particularly when he was in his hey-day. The runner being linked to doping allegations were all too common and so it is impossible to view his career as entirely unblemished.

It should be noted from the off that Farah never tested positive for anything. Not once did he fail a doping test and never was hesuspended for any doping violations.

Yet there is a shadow over his career which, at one point, was dark enough to prompt BBC’s current affairs programme, Panorama, to investigate him, although nothing definitively incriminating, or incriminating enough to see him banned, was found.

The first scandal that engulfed him came ahead of the 2012 Olympics; he missed a doping test after apparently “not hearing his doorbell ring” when the testers called at his house. It was his second missed test in two years.

Then, in the summer of 2017, when the Fancy Bears hacking group released information from the database of the IAAF, the sport’s global governing body, it reportedly showed that Farah had, the previous year, recorded what is known as “atypical” values on his Athlete Biological Passport, which tracks an athlete’s blood values and can give an indication of doping.

It emerged that an IAAF official had noted that his values were far from normal, writing beside his name: “Likely doping; Passport suspicious: further data is required”. In the end, Farah was cleared of wrongdoing after a separate leaked spreadsheet said his records had been “now flagged as normal with the last sample”, but it is not the kind of attention any Olympic champion would welcome.

And that was not the end of the doping controversies.

The Englishman was a long-time member of the Nike Oregon Project, a training group based in the United States, led by the now infamous coach, Alberto Salazar.

The American coach is serving a four-year ban for doping offences which included tampering with doping control methods and trafficking testosterone. The Nike Oregon Project was closed down in the aftermath, but the close association with Salazar wasn’t a great one for Farah when these revelations emerged.

But perhaps most incriminatingly for Farah, were the allegations at the centre of the Panorama investigation; that he had taken, and then lied about taking, L-carnitine injections.

While not prohibited, the performance-enhancing L-carnitine is controversial and Farah did his reputation no favours by “forgetting” he had taken these injections in 2015.

On the track, Farah is unquestionably one of Britain’s greatest sportspeople. But the murkiness of his actions off the track make it impossible for me to look at him in an entirely positive light as he waltzes off into retirement.

This may be unfair because Farah has not been charged as a doping cheat. And he has strenuously denied each and every allegation and accusation of doping, saying he has “no tolerance” of anyone who breaks the rules. 

But the shadow cast by so many allegations cannot be ignored.

Only time will tell how kindly history judges Farah. His medal collection is unrivalled except for a select few individuals, but there has been a lot of smoke, if no fire.

Any athlete who has followed Farah’s career should take heed that however much success one can garner, any reputation can be tarnished if they are involved in doping allegations one too many times.