Androstenedione is an endogenous androgen steroid hormone and intermediate in the biosynthesis of testosterone from dehydroepiandrosterone (DHEA). In turn, Androstenedione is also a precursor of dihydrotestosterone (DHT), estrogens such as estradiol and estrone, and the neurosteroid 3a-androstanediol. Androstenedione is used to increase the production of the hormone testosterone to enhance athletic performance, increase energy, keep red blood cells healthy, enhance recovery and growth from exercise, and increase sexual desire and performance.

Androstenedione has been shown to increase serum testosterone levels over an eight-hour period in men when taken as a single oral dose of 300 mg per day, but a dose of 100 mg had no significant effect on serum testosterone. However, serum levels of estradiol increased following both the 100 mg and 300 mg doses. The study also reported that the serum level of estrogens and testosterone produced varied widely among individuals. Androstenedione is currently used as a nutritional supplement to grow bigger muscles and stronger bones. This implies that androstenedione may have anabolic properties. Even though it has not been convincingly demonstrated yet that androstenedione is an anabolic steroid, its anabolic properties are likely based on its proven ability to increase testosterone levels. The role of testosterone in building stronger muscles and bones is widely accepted. Thus, high doses of testosterone-boosting drugs combined with strength training have been shown to increase muscle size and strength even in normal young men. This confirms what thousands of athletes who take anabolic steroids have known for decades.

Yet androstenedione is different from testosterone-boosting drugs in a number of important aspects. To begin with, androstenedione is a naturally occurring substance that is produced by the body itself. In contrast to synthetic anabolic steroids, androstenedione is right at home in the human body, and perfectly complements the complex hormonal network in the body. Information about possible side effects and risks of androstenedione is very limited. Also, recent studies show that the drug's actions don't support manufacturer's claims. While a few individuals have shown increased levels of testosterone, most failed to achieve increases in blood testosterone levels. Initial medical research has raised concerns about this supplement's safety. Doctors worry that androstenedione may increase the risk of heart disease or liver cancer. In addition, research also associates androstenedione use with increases in estradiol, a female estrogen.

Androstenedione:
https://drugs.ncats.io/drug/409J2J96VR

Andro is a hormone that became a star in the bodybuilding supplement industry in the nineties. The substance, a natural forerunner of both testosterone and estrogen, made headlines in 1998 when a reporter saw a bottle of the stuff in the locker of baseball star Mark McGwire. Encouraged by wild claims that andro could boost their testosterone levels by as much as 400 percent, would-be power hitters and athletes of many other stripes started taking the supplements in hopes of matching McGwire's bulk and strength.

In October 2004, President Bush signed into law the Anabolic Steroid Control Act, which added andro to the list of banned, non-prescription steroid-based drugs. Major League Baseball, the National Football League, the Olympics, and the National Collegiate Athletics Association all now prohibit the use of andro.

When the andro craze took off in 1998, nobody really knew how the supplements affected the body. Now, thanks to a landmark study released in 1999, scientists have some answers. And they're not the answers that andro users want to hear.

McGwire may have hit 70 home runs in 1998, but a study from Iowa State University suggests androstenedione was just a bystander. Researchers tested the supplements on 20 healthy men as they undertook eight weeks of weight training. Ten of them, selected at random, took 300 milligrams of andro (slightly more than manufacturers generally recommend) each day for six weeks, while the other 10 unknowingly took a dummy pill made of rice flour. Throughout the training period, the two groups enjoyed the same gains in muscle bulk and strength. Either rice flour deserves a craze of its own, or andro is a bust.

Andro failed to live up to its billing in another important way: It didn't budge the subjects' testosterone levels. It did, however, increase their supply of estrogen, hardly the goal most male users have in mind. The supplement may have a different effect on women. One study of two women found that a single 100-mg dose of andro briefly increased their testosterone levels. The Iowa State researchers speculate that andro may raise testosterone levels only in people whose bodies don't already have plenty, namely women and elderly men. And although none of the subjects showed side effects from their revved-up estrogen levels, it's conceivable that any man who took the supplement long enough would risk growing breasts. Good luck finding that on a warning label.

Excerpt from healthday.com:
https://consumer.healthday.com/encyclopedia/exercise-and-fitness-18/misc-health-news-265/androstenedione-andro-buyer-beware-647218.html

Androstenedione is a steroid hormone that has weak, androgenic actions on the body itself. However, it mainly acts as a stepping stone in the manufacture of testosterone and oestrogen within the body. Androstenedione is described as a 'pro-hormone' because it has few effects itself. Instead, it is important because of the ability of different parts of the body to convert it into the hormones, testosterone and oestrogen, which exert many effects on the body. In females, the outer part of the adrenal glands (known as the cortex) and the ovaries release androstenedione into the bloodstream where it is converted to provide around half of all testosterone and almost all of the body's oestrone, a form of oestrogen. Although the testes produce large amounts of androstenedione in males, they secrete little of this into the blood and, instead, rapidly convert it into testosterone within the testes. The adrenal glands also produce androstenedione in men, but this contribution is swamped by the testes' overwhelming production of the other androgenic hormone, testosterone.

Excerpt from yourhomones.info:
http://www.yourhormones.info/hormones/androstenedione/

• Androstenedione is a weak androgen steroid hormone
• It's is a precursor of testosterone and other androgens, as well as of estrogens like estrone, in the body.
• It's the common precursor of the androgen and estrogen sex hormones.
• It's closely related to androstenediol

• Adrenarche:
  In children aged 6 to 8 years old, there is a rise in androstenedione secretion along with DHEA during adrenarche. This rise in androstenedione and DHEA is hypothesized to play a crucial role for learning social, cultural and ecological skills, such as the development and understanding of sexual attraction. Furthermore, it is thought that androstenedione plays a role in levels of aggression and competition in boys, as a positive correlation between the two were observed, while testosterone levels were below detection.
• Use as a supplement:
  Androstenedione was manufactured as a dietary supplement, often called andro or andros for short. Sports Illustrated credits Patrick Arnold with introducing androstenedione to the North American market. Androstenedione was legal and able to be purchased over the counter, and, as a consequence, it was in common use in Major League Baseball throughout the 1990s by record-breaking sluggers like Mark McGwire.
• Regulation:
  Androstenedione is banned by the World Anti-Doping Agency, and from the Olympic Games. The International Olympic Committee in 1997 banned androstenedione and placed it under the category of androgenic-anabolic steroids. Androstenedione is banned by MLB, the NFL, USOC, NCAA, and by the NBA.

On March 12, 2004, the Anabolic Steroid Control Act of 2004 was introduced into the United States Senate. It amended the Controlled Substance Act to place both anabolic steroids and prohormones on a list of controlled substances, making possession of the banned substances a federal crime. The law took effect on January 20, 2005. However, androstenedione was legally defined as an anabolic steroid, even though there is scant evidence that androstenedione itself is anabolic in nature. On April 11, 2004, the United States Food and Drug Administration banned the sale of androstenedione, citing that the drug poses significant health risks commonly associated with steroids. Androstenedione is currently banned by the U.S. military.