The right dose of a dietary supplement used by some athletes can boost levels of a male sex hormone, research has found.

The Journal of the American Medical Association reported on a project at Massachussets General Hospital examining claims made about androstenedione.

The researchers found that, in men, the amount of testosterone, a hormone associated with aggression, was boosted by taking 300mg a day.

But the supplement also more than doubled the natural levels of two female sex hormones in the men's blood.

However, the study did not look into the long-term safety of the drug, or any other physical changes - desireable or unexpected - associated with its use.

Elevated testosterone levels are thought to lower levels of so-called "good cholesterol" and can have masculinizing effects on women, such as increased body hair and deepening voice.

Conversely, young people who have elevated levels of oestrogens could experience feminising effects such as breast growth.

And levels of the two female sex hormones were also increased by taking the supplement.

Strength and stamina

Androstenedione is widely available and marketed primarily to athletes and bodybuilders with claims that it will increase strength, stamina and muscle mass.

It received wide public attention in 1998 when Mark McGwire, the US baseball player who scored a record number of home runs in one season admitted using it as part of his training programme.

The study looked at 42 healthy young men who had never before taken androstenedione, and gave them varying doses of the drug.

While a 100mg dose had no appreciable effect on testosterone levels, the 300mg doses increased levels by an average of 34%.

In one third of those taking the 300mg dose, testosterone levels exceeded the normal range for men.

However, increases in female hormones were more marked. Estrone increased 74% at 100mg and 196% at 300mg, and estradiol by 42% at 100mg and 128% at 300mg.

The amount of the hormone in the blood returned to normal only a day after the patients stopped taking it.

Dr Joel Finkelstein, of the Massachusetts General Hospital Endocrine Unit, said: "A lot of people have been taking androstenedione under the assumption that it will raise their testosterone.

"This is the first study to show that sufficient doses do raise serum testosterone.

"But we now need to to on and study whether that increase actually translates into changes in athletic performance or into negative health effects."