Accidentally Took a Double Dose of Tamsulosin (Flomax)? What Happens Next

If you took your tamsulosin twice today, you have most likely taken 0.8 mg instead of 0.4 mg. That is not an unknown dose. The US label for Flomax allows 0.8 mg once a day for men who do not respond to 0.4 mg, and nearly 500 men took it every day in the trials behind the approval. The realistic risk is not poisoning. It is dizziness or fainting when you stand up, over the next several hours, and it is a risk you can manage.

This page is for the usual mistake: one extra 0.4 mg capsule, in an adult man who takes tamsulosin for his prostate. If more was taken, if a child or someone who is not prescribed tamsulosin swallowed it, or if you feel faint now, skip to the section on when to get help.

What to do right now

Do not take anything to make up for it, and do not try to make yourself vomit. The capsules are built to release slowly, so there is nothing to gain from it.

Sit or lie down if you feel light-headed, and get up slowly for the rest of the day: sit on the edge of the bed for a minute before standing, especially at night.

Do not drive or use machinery until you know how you feel. The label warns about dizziness and driving even at the normal dose.

Skip alcohol today, and do not take sildenafil, tadalafil or another erectile dysfunction drug today. The label calls for caution combining these with alpha blockers because both lower blood pressure.

Call your prescriber or pharmacist. The Flomax patient leaflet says exactly that: if you take more capsules than prescribed, call your doctor right away. In the US you can also call Poison Control at 1-800-222-1222, which is free and answers around the clock.

Why one extra capsule is usually not dangerous

Tamsulosin was tested at 0.4 mg and 0.8 mg side by side. In the two US trials behind the label, dizziness was reported by 15 percent of men on 0.4 mg, 17 percent on 0.8 mg and 10 percent on placebo. Symptomatic low blood pressure on standing was reported by 1 of 502 men on 0.4 mg and 2 of 492 on 0.8 mg. Fainting was reported by 1, 2 and 3 men respectively, the last figure being the placebo group.

So the higher dose adds a few percentage points of dizziness, not a new kind of risk. One caveat matters: in those trials, men on 0.8 mg took 0.4 mg for a week first. Your body did not get that warm-up, so treat today the way you would treat the first dose.

The hours to watch

Tamsulosin reaches its highest level in the blood four to five hours after a dose on an empty stomach, and six to seven hours after a dose taken with food. That window is when dizziness on standing is most likely. Its half-life in men with prostate enlargement is 14 to 15 hours, so the extra dose has largely faded by the next day.

Food matters more than most people think. In the label's own pharmacokinetic table, men taking 0.4 mg after a light breakfast had an average peak of 10.1 ng/mL. On an empty stomach it was 17.1. Men taking 0.8 mg on an empty stomach reached 41.6, about four times the usual level. If you doubled up on an empty stomach, eat something and take the rest of the day carefully. This is also why the label asks for tamsulosin about 30 minutes after the same meal each day.

Age adds to it. The label reports about 40 percent higher overall exposure in men aged 55 to 75 than in men aged 20 to 32, from the same dose.

Who should call their prescriber today rather than wait and see

You have just started tamsulosin, or restarted it after a break of several days. The drop in blood pressure on standing is most marked at the start and when the dose changes.

You take blood pressure medication, another alpha blocker such as doxazosin, terazosin or alfuzosin, or an erectile dysfunction drug.

You take a drug that slows tamsulosin's breakdown. The label names strong CYP3A4 inhibitors such as ketoconazole, which should not be combined with tamsulosin at all, and urges caution with erythromycin, paroxetine and terbinafine. These raise tamsulosin levels on top of the double dose.

You were already on 0.8 mg and took it twice, which is 1.6 mg. That is above the dose range the label's pharmacokinetic studies covered, which went up to 1 mg.

What a real overdose looks like

The two overdoses described in a Canadian tamsulosin product monograph give a sense of scale. One person took 5 mg, the equivalent of about twelve 0.4 mg capsules, and had a systolic blood pressure of 70 mm Hg with vomiting and diarrhoea. They were treated with fluids and could go home the same day. Another took thirty 0.4 mg capsules, 12 mg in total, and reported a severe headache that resolved the same day.

Those are case reports, not a guarantee, and they are many times the amount in a double dose. They show what the danger is: blood pressure, not organ damage.

Is there an antidote for tamsulosin?

No. There is no specific antidote. The label describes the treatment of an overdose that causes low blood pressure: lying the person flat, then intravenous fluids if that is not enough, then drugs that raise blood pressure if needed, while kidney function is monitored. Dialysis is unlikely to help, because 94 to 99 percent of tamsulosin in the blood is bound to proteins.

That is hospital care for a large overdose. For one extra capsule, lying down when you feel dizzy is the same principle on a smaller scale.

When to get emergency help

Call 911, or your local emergency number, if you faint, if you feel faint even when lying down, if you have chest pain, a racing or irregular heartbeat, confusion or trouble breathing.

Get urgent care for an erection that lasts more than four hours. Priapism is rare with tamsulosin, the label estimates less than 1 in 50,000 men, but it can cause permanent damage if it is not treated quickly.

If a child, a woman, or anyone not prescribed tamsulosin swallowed a capsule, or if you are unsure how many were taken, call Poison Control at 1-800-222-1222 now rather than waiting for symptoms.

Tomorrow, and how to avoid a repeat

The labels do not tell you to skip the next dose to make up for a double one, and they do not give any rule for the day after. Ask your prescriber or pharmacist when you call whether to take tomorrow's dose at the usual time.

The rules for the opposite mistake are clear. If you miss a dose, take it the same day when you remember. If you miss a whole day, skip it and carry on as normal. Never take two doses at once to catch up, which is how many double doses happen. If you miss several days, talk to your prescriber before restarting, because the label says to restart at 0.4 mg.

A weekly pill organizer settles the question of whether today's dose has been taken, which is the uncertainty behind most double doses. Taking it after the same meal every day helps for the same reason.