With the naked eye it looks like Juan Francisco Villa has lots of hair, but under a microscope you can see where it's thinning.

Juan Francisco Villa: "One day I was just combing my hair and a big clump came out, and I was a little worried about it."

Hair restoration specialist Dr. Paul McAndrews prescribed the baldness drug Propecia.

Dr. Paul McAndrews: "Propecia is the most effective medical therapy for hair loss."

Propecia is a smaller dose of the same drug given to treat an enlarged prostate.

PSA is the screening test for prostate cancer. Oncologist Dr. Simon Tchekmedyian says when a man stops or starts taking Propecia, "It will have some effect on the prostate . And one of the effects is that it lowers the PSA levels by about 40 to 50 percent."

PSA levels can appear to plummet or rise which can send the wrong signal to your doctor.

"Otherwise the patient could end up having a biopsy or some other procedure when actually it was not necessary. Or the converse, a cancer may be missed."

Dr. McAndrews always warns his patients to tell their doctors they're on Propecia, but he says the drug is completely safe and one study on the larger dose of Finesteride revealed it might even benefit the prostate.

"What we saw was almost a 25 percent decrease risk of prostate cancer," said Dr. McAndrews.

But Dr. Tchekmeydian says the same study also showed a slight increase in a very aggressive form of prostate cancer.

So Finesteride is not generally considered an anti-prostate cancer drug.

The bottom line: Tell your doctor you're on Propecia and get informed and educated.

"Anything I consume, I'm very wary about it so knowing what I'm taking is important to me," said Villa.

Again, both doctors say Propecia has been thoroughly tested for safety and efficacy. And if you tell your doctor when you started or stopped the drug it can help him or her accurately interpret your test results.