We all see a parade of guys with prostates the size of grapefruits (or at least, that’s how they describe the feeling when trying to pass water at 3 AM). And for many of them, particularly those with significantly enlarged glands, Proscar (finasteride 5mg) becomes a long-term companion. It’s one of the mainstays for actually shrinking the prostate, unlike alpha-blockers which just relax the muscle. Proscar gets in there and, by inhibiting 5-alpha reductase, reduces dihydrotestosterone levels within the prostate, leading to a gradual reduction in volume. We’re talking a 20-30% shrinkage over 6-12 months, sometimes more. That can make a real difference to flow, to nocturia, to overall quality of life.
But here’s the rub, and it’s a big one that I still see catching people out: Proscar messes with PSA levels. Specifically, it tends to lower them by about 50% after about 6-12 months of consistent use. And if you’re not accounting for that, you can get a very false sense of security when it comes to prostate cancer screening.
I had a classic case of this just last year. Mr. Henderson (not his real name, but you get the gist), a delightful chap in his late 60s, had been on Proscar for about five years. Started him on it because his prostate was hefty, and his symptoms were really bothering him. The Proscar, along with an alpha-blocker initially, worked wonders. His flow improved, he was sleeping better. We monitored his PSA. Before Proscar, it was around 3.5 ng/mL. After a year on Proscar, it settled down to a very respectable 1.7 ng/mL. Perfect. Doubled, that’s 3.4, right back where he started, but with a smaller, happier prostate.
For the next few years, his PSA, when checked, hovered around 1.6 to 1.8 ng/mL. All good. Then he comes in for his annual, and he’s got a printout from a recent check-up with his GP. PSA: 2.2 ng/mL. His GP had, quite reasonably from their perspective, seen 2.2 and thought, "That's still pretty good for his age, well within normal limits." And Mr. Henderson was chuffed. "Lowest it's been in ages before the Proscar, Doc!" he said, beaming.
My internal alarm bells didn’t just ring; they practically played a full orchestral piece. A PSA of 2.2 ng/mL in a man who has been stable on Proscar for years, whose PSA should be roughly halved by the medication? That means his "true" PSA, the level it would likely be if he wasn't on Proscar, is closer to 4.4 ng/mL. And more importantly, it represented a significant rise from his established Proscar-treated baseline of 1.7. That jump from 1.7 to 2.2, when doubled, is a jump from a conceptual 3.4 to 4.4. That’s a rising PSA, and that always needs investigating.
So, gently, I had to explain this to Mr. Henderson. "That 2.2 is good on paper," I said, "but because you're on Proscar, we have to look at it differently. The Proscar is like wearing special glasses that make the number look lower than it really is, compared to someone not taking it. And your number, even with the glasses, has gone up."
He was a bit crestfallen, naturally, but understood. We had a long chat. Did a digital rectal exam – his prostate felt a bit firmer in one area than I remembered. So, we proceeded to an MRI, which showed a PIRADS 4 lesion. Biopsy confirmed a Gleason 7 (3+4) cancer. Thankfully, it appeared localized. He’s since had treatment and is doing well.
But it’s a stark reminder: when a patient is on Proscar (or dutasteride, for that matter), you must interpret their PSA in that context. A stable, low PSA on Proscar is reassuring. A rising PSA, even if the absolute value still looks "low" or "normal" for their age, is a red flag. The general rule of thumb is to double the PSA value for men who have been on Proscar for at least six months to get an estimate of what it might be without the drug. But even more critical is watching the trend from their established baseline on the medication. Any persistent rise from that nadir warrants attention.
Proscar is a great drug for BPH. It can genuinely improve symptoms and can even reduce the risk of acute urinary retention and the need for BPH surgery down the line. But it requires diligent, informed follow-up. It's not a "prescribe and forget" medication. We need to educate our patients about this PSA effect, and we need to be vigilant ourselves. That little detail can be the difference between catching something nasty early, or missing a crucial window. Don't let the Proscar effect fool you!
And if you want more detailed and professional information, here’s the link: https://www.imedix.com/drugs/proscar/