Androgens, Anti-androgens, and Treating Erectile Dysfunction
Testosterone is the male counterpart of the story, and its pharmacology runs both ways: sometimes we replace it, and sometimes we block it — including to slow prostate cancer, which feeds on it. Add the famous erectile-dysfunction drugs, whose mechanism is a small masterpiece of physiology, and you have a compact, high-yield chapter.
Testosterone is the main androgen — the hormone behind male development, muscle, libido and sperm production. Its pharmacology comes in three practical pieces. Sometimes we give it as replacement, when the body doesn't make enough. Sometimes we deliberately block it, most importantly because prostate cancer is fuelled by testosterone, so lowering or blocking the hormone can slow the disease. And separately, there are the drugs for erectile dysfunction — the well-known 'PDE5 inhibitors' — which don't touch testosterone at all but work through the plumbing of blood flow. Three neat topics, one hormone family.
Replacing and blocking testosterone
Give it when it's lacking; block it when it drives disease. Testosterone replacement is given to men whose testes or pituitary fail to produce enough (male hypogonadism), relieving low libido, fatigue and loss of muscle and bone. It must be used carefully — it's avoided in prostate and breast cancer, and it can thicken the blood — and, like any exogenous hormone, it suppresses the body's own production by feedback (which is one reason anabolic-steroid abuse for muscle-building shrinks the testes). The opposite strategy, anti-androgen therapy, is central to treating prostate cancer, which grows in response to testosterone. There are two ways to cut off that fuel: lower the hormone's production, or block its receptor. Production is lowered with GnRH agonists/antagonists that switch off the testosterone-driving signal from the brain; the receptor is blocked with anti-androgen drugs (like bicalutamide). A related, milder blocker is finasteride, a 5-alpha-reductase inhibitor that stops testosterone being converted to its more potent form (DHT) in specific tissues — used for benign prostate enlargement and male-pattern hair loss. Give testosterone, or take it away — the direction depends entirely on the problem.
Erectile dysfunction: the PDE5 inhibitors
The erectile-dysfunction drugs — sildenafil (Viagra), tadalafil and the rest, the 'PDE5 inhibitors' — are a lovely lesson in physiology. An erection happens when nitric oxide relaxes the smooth muscle of the penis, letting blood flow in; it does this by raising a messenger molecule called cGMP. An enzyme, PDE5, normally breaks cGMP down and ends the response. These drugs block PDE5, so cGMP lasts longer and the relaxation is enhanced — but only when there's sexual stimulation to start the nitric oxide signal in the first place. That's why they enhance a natural erection rather than causing one out of nowhere. The one crucial safety rule flows directly from the mechanism: PDE5 inhibitors must NEVER be combined with nitrates (the angina drugs from the cardiovascular chapter), because nitrates also work through this same nitric-oxide/cGMP pathway. Stack the two and you can cause a catastrophic, life-threatening drop in blood pressure. Sildenafil plus nitrate is one of the classic dangerous drug combinations — remember it as firmly as you remember what these drugs are for.
- Testosterone replacement treats male hypogonadism; avoid in prostate/breast cancer; suppresses own production.
- Prostate cancer is testosterone-driven → treat by lowering it (GnRH drugs) or blocking its receptor (anti-androgens).
- Finasteride (5-alpha-reductase inhibitor) blocks conversion to DHT — for benign prostate enlargement & hair loss.
- PDE5 inhibitors (sildenafil) enhance erection via the nitric oxide/cGMP pathway — need sexual stimulation.
- NEVER combine PDE5 inhibitors with nitrates — dangerous, life-threatening hypotension.
The sildenafil–nitrate rule is one of the most important drug-interaction facts in all of medicine, and it makes perfect sense once you see that both drugs pull the same lever. Nitrates release nitric oxide to raise cGMP and relax blood vessels (relieving angina); PDE5 inhibitors stop cGMP being broken down, so it accumulates. Give them together and you get a massive, uncontrolled rise in cGMP throughout the vasculature — vessels everywhere dilate at once and blood pressure collapses. It's a textbook example of two drugs that are individually useful becoming dangerous because they act on different points of the same pathway. Whenever two drugs push the same physiological button from different angles, ask whether their effects will add up to something extreme.
- Combining a PDE5 inhibitor with a nitrate — profound, dangerous hypotension.
- Giving testosterone in prostate cancer — it fuels the tumour.
- Expecting a PDE5 inhibitor to work without sexual stimulation — it only enhances the natural response.
- Forgetting anabolic steroid abuse suppresses natural testosterone and shrinks the testes.
Which combination is dangerous and must be avoided?
- Testosterone: replace in hypogonadism (avoid in prostate/breast cancer); it suppresses own production.
- Prostate cancer is testosterone-driven → lower it (GnRH drugs) or block the receptor (anti-androgens).
- PDE5 inhibitors (sildenafil) enhance erection via NO/cGMP — need stimulation.
- NEVER give a PDE5 inhibitor with a nitrate — catastrophic hypotension.
- Katzung BG. Basic & Clinical Pharmacology — Androgens, Antiandrogens & Drugs for Erectile Dysfunction.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Androgens & antiandrogens.
- AUA / EAU — Guidelines on erectile dysfunction & prostate cancer hormonal therapy.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Androgens & PDE5 inhibitors.
