Feb. 19, 2001 -- They certainly don't recommend it as a means of birth control , but researchers say marijuana may limit the likelihood that pot-smoking couples will conceive a child.

Cannabis has been reported to reduce sperm count , but a recent study conducted at the University at Buffalo-SUNY shows that the illegal substance also may inhibit sperm's ability to fertilize an egg.

"If you are concerned about being a dad and smoking marijuana, this is something that you should take into account," says Herbert Schuel, PhD, lead author of the research unveiled late last year at the American Society of Cell Biology annual meeting in San Francisco.

The effect of marijuana has been the subject of numerous studies, including many regarding reproduction. Some show a reduced sex drive with use. Others say the substance disrupts a woman's hormonal cycles. But are these studies reliable, or just "reefer madness" myths?

"If you ask me if this is based on good scientific data, no, it's not," says Lester Grinspoon MD, professor emeritus of psychiatry at Harvard Medical School.

Grinspoon has been studying the effects of marijuana for more than 30 years, beginning when his son used it to ease nausea caused by chemotherapy treatments. He says many studies regarding cannabis and childbirth fail to take into account socioeconomic factors, degree of prenatal care, or behaviors like smoking tobacco.

Grinspoon, author of Marihuana Reconsidered and Marihuana -- the Forbidden Medicine, says he is familiar with Schuel's findings but remains skeptical. With more than 70 million people who have tried cannabis and an estimated 12 million who are regular users, he says the evidence should be more widespread.

"The fact of the matter is people have difficulty accepting the fact that cannabis is one of the least toxic substances known to man," Grinspoon says. "We keep searching with these microscopic lenses for things, but it just doesn't show."

Eric A. Voth, MD, FACP, disagrees. Voth, chairman of the Institute on Global Drug Policy (an alliance of physicians, lawyers, and drug specialists formed recently by the Drug Free America Foundation) says that while 30 years ago, studies on marijuana may have been flawed, today's research is trustworthy. And hopeful parents should take note.

"Significantly heavy use will reduce the actual sperm counts, and then there is the secondary effect," says Voth, an internal medicine and addiction medicine specialist in Topeka, Kan. "Some people say it is an aphrodisiac, but it can really reduce sexual performance and testosterone ."

Using human sperm, Schuel's team of researchers from Buffalo, the University of Connecticut, Eastern Virginia Medical School, and the University of California at Irvine, showed that anandamides -- marijuana-like compounds -- exist in men's and women's reproductive fluids that sperm are exposed to as they move from the vagina to the fallopian tube. A high concentration of anandamide slows the sperm's swimming and may inhibit the sperm from binding to the egg's coat. Anandamide and THC -- the substance responsible for the "high" produced by marijuana -- also regulated changes to the sperm's structure during the process of preparing them to penetrate the egg's coat, according to the study, funded by the University at Buffalo-SUNY and the National Institute on Drug Abuse.

There is no evidence that the effect is irreversible, but Schuel says the more someone smokes, the longer it could take to become pregnant.

"Cannabinoids in marijuana are extremely soluble in fat, which means they accumulate and are stored in body fat," Schuel says. "The longer a person smokes, the more of these compounds are going to store in their bodies. If someone were going to stop, how long is it going to take for the storage of drugs to wash out? That could be several months if you are a heavy user."

Marijuana is the most commonly used illicit drug, according to federal statistics. The most prevalent time for people to experiment with it happens to coincide with their childbearing years. Almost half of all Americans ages 18 to 25 have used marijuana at some point in their lives, and about 15% are current users. Approximately 3% of them are pregnant, according to the statistics.

In addition to fertility issues, marijuana has been linked in some studies to children's low birth weight, behavioral problems, poor growth, physical abnormalities, lower IQ, and difficulty with language comprehension and memory. One study, conducted by researchers at the University of Minnesota and published in May 1989 in the journal Cancer, found that exposing a fetus to marijuana increased a child's risk of getting leukemia. So at a time when women are urged to avoid certain legal substances like antihistamines or caffeine, it is no wonder that many are encouraged to add marijuana to their list of pregnancy no-nos.

"The reality is that THC crosses the placenta virtually 100%," says Voth. "That is a huge concern."

Nancy L. Day, PhD, a professor of psychiatry and epidemiology at the University of Pittsburgh School of Medicine, has been studying a group of children prenatally exposed to cannabis for the last 10 years. Her work, published in the May-June 2000 issue of Neurotoxicology and Teratology, indicates that such children tend to be more hyperactive, inattentive, and impulsive.

"It is important to know that though subtle, that impact is there," Day tells WebMD. "It's not a big deal, but it is a concern."

Additional studies are needed to confirm recent findings, says S.K. Dey, PhD, professor of molecular and integrative physiology at the University of Kansas Medical Center, whose own research has shown that cannabis can interfere with embryonic development in mice. Most studies regarding marijuana focus on neurological effects, rather than its reproductive ones, he says.

"There is not any hard data yet," Dey says.

Despite a lack of solid proof, even the National Organization for the Reform of Marijuana Laws (NORML) agrees that women who are pregnant or those trying to conceive should take a break from getting high.

"There is not an iota of research that smoking harms the fetus, but I do think physicians would urge, 'Don't use any drugs,'" says Keith Stroup, NORML's executive director. "Common sense tells us it is smarter to avoid it."

Kimberly Sanchez is a St. Louis freelance writer who has written for the Los Angeles Times, New York Newsday, the Chicago Sun-Times, and the Dallas Morning News. She is a frequent contributor to WebMD.