Perhaps the warning signs were there for tennis player Maria Sharapova before she failed a drugs test for using meldonium.
The substance was added to the World Anti-Doping Agency's (Wada) list of prohibited substances on 1 January 2016, with five-time Grand Slam champion Sharapova failing a test at the Australian Open later that month.
However, meldonium, which is thought to improve an athlete's stamina and endurance, featured on Wada's watch list in 2015 - meaning it was not banned but was being monitored "in order to detect patterns of misuse in sport".
For a substance to be added to Wada's prohibited list involves a three-stage consultation process, which lasts several months and includes input from Wada experts, its health, medical and research committee (HMRC) and its executive committee.
According to Wada, a substance may be "considered" for the prohibited list if it meets two of the following three criteria:
- Enhances performance
- Poses a threat to athlete health
- Violates the spirit of sport
A substance can be added to the prohibited list without first featuring on the watch list.
Meldonium - or mildronate as it was known to 28-year-old Russian Sharapova - is no longer on the 2016 watch list, but several other substances remain on it.
Some are monitored only during competition, some outside of competition and some both in and out of competition.
Which substances are on Wada's 2016 watch list?
- Bupropion - anti-depressant
- Caffeine - banned for 20 years until 2004
- Phenylephrine and phenylpropanolamine - decongestants
- Pipradrol - counters fatigue
- Synephrine - found in weight-loss and energy products
- Mitragynine - natural occurring plant compound said to aid recovery
- Tramadol - strong painkiller
Glucocorticoids - anti-inflammatories
Telmisartan - used for high blood pressure
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