Marijuana users have insisted for years that the drug does wonders for their love life, while scientists argued that it depresses sexual desire. Now it appears both are correct, a new study says.

Animal research indicates marijuana's aphrodisiac effect is a factor of dose and timing, University of Texas researchers have concluded.

The active ingredient in marijuana, tetrahydrocannabinol (THC), has a seesaw effect on male sexual hormones in a short period of time, the scientists say. THC first boosts levels of testosterone and other sex hormones in mice, which are considered a good animal model with marijuana for humans. But after a short time, these levels plummet far below normal, said Dr. Susan Dalterio.

The male mice receiving low THC doses maintained high levels of testosterone, a hormone produced primarily in the testes, for more than an hour, she said. However, mice getting high doses show a quick increase, then a dramatic drop in testosterone to below normal levels in 20 minutes.

Dalterio previously reported that copulatory behavior in male mice dropped sharply after they got THC. But she found recently that the mice perform well "if the female is introduced immediately, rather than one to four hours after drug administration. . . . A little bit [of THC], without waiting too long, can enhance sexual performance," she said.

Dalterio said chronic marijuana users, those who smoke three to five marijuana cigarettes a day, report impaired sexual functioning. These people are roughly equivalent to high-dose mise in studies, she said. The low-dose mouse group that got initial sexual enhancement with THC are more like people who have one marijuana cigarette initially, she added.

In females, the sexual effects of marijuana apparently have less to do with hormones than is the case with males. Animal studies at UCLA and other institutions, Dr. Dalterio said, show marijuana enhances female reproductive behavior by affecting the brain and reducing inhibitions.