Spironolactone

Spironolactone is a tablet originally used for blood pressure, heart failure and fluid retention that also blocks testosterone. It is the most common [anti-androgen](/glossary/anti-androgen) for feminizing hormones in the US. Often shortened to "spiro".

Where you'll meet it

On most US feminizing prescriptions that include a blocker, and in comparisons with cyproterone, which has filled the same role in the UK and Europe, and with the GnRH agonist injections now common in UK gender services. Outside the US it is prescribed less often for this purpose, though it is used. You may also know it from other contexts: dermatologists prescribe it for acne and hair problems in cis women.

How it works

It works two ways at once. It partly blocks testosterone from attaching to cells, and it partly reduces how much testosterone is made. Neither effect is very strong, which makes it a relatively mild anti-androgen. Some people on it still have testosterone above the level their prescriber is aiming for, commonly given as under about 50 ng/dL (under about 1.7 nmol/L). That is one reason some prescribers move to a different blocker or to estrogen alone, called monotherapy.

Because part of its effect is blocking rather than lowering, a blood test does not capture the whole picture, but testosterone levels are still the main way prescribers judge whether it is doing enough.

The side effects people notice

• Needing to pee more often. It is a diuretic, so this is almost universal, especially in the first weeks. Many people take it earlier in the day for that reason; ask your prescriber how they want you to take it.
• Light-headedness when standing up, because it can lower blood pressure. Thirst and a tendency to dehydrate come with it, which matters in hot weather and after exercise or alcohol.
• Raised potassium. It makes the body hold on to potassium. For most healthy people this stays within a safe range, but a high level can affect the heart, so potassium and kidney function are on the blood tests of people who take it.

Why potassium is on your blood tests

The risk goes up with kidney problems and with other things that raise potassium: potassium supplements, potassium-based salt substitutes, and some blood pressure medicines. Your prescriber will ask about these. Mention anything new you start, including over-the-counter products. Monitoring usually follows the general pattern of bloods roughly every 3 months in the first year, then once or twice a year once stable; blood tests and monitoring has the detail. Muscle weakness, an irregular heartbeat or feeling very unwell are reasons to get checked promptly, not at your next routine appointment. Feminizing risks covers warning signs.

Where it fits

It is cheap, often under $20 a month self-pay in the US, and widely available, and for many people it does the job with little trouble. Cost and insurance has the wider picture. After an orchiectomy, most people stop it under their prescriber's guidance, because there is little testosterone left to block.

Common questions

"Spiro does the feminizing." It does not; estrogen does. Spironolactone clears the way by getting testosterone out of the picture.

"More spiro means faster changes." Past what suppresses testosterone, more mainly adds side effects. Dose changes belong with your prescriber and a blood test.

What to ask

Ask what testosterone level your prescriber is aiming for on spironolactone, how often potassium and kidney function will be checked, and what they would switch to if it is not enough. Anti-androgens compares it with the alternatives.

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