Cialis (Tadalafil) Side Effects and Dosing: 10 mg, 20 mg, and Daily 5 mg Explained (2026)

The short version

Tadalafil (brand name Cialis) treats erectile dysfunction in two different dosing modes: an as-needed tablet, usually 10 mg taken before sex and adjusted between 5 and 20 mg, and a lower once-daily tablet, 2.5 or 5 mg, taken every day regardless of timing [1]. Its defining trait is how long it works: it improves erectile function for up to 36 hours after a dose, which is why people call it the weekend pill [1]. Most side effects are mild and predictable, including headache, indigestion, flushing, a stuffy nose, and a back or muscle ache that shows up more with tadalafil than with other pills in its class [1]. Serious problems are rare but real: an erection lasting more than four hours, sudden vision loss, and sudden hearing loss all mean stop and get help [1]. The one non-negotiable rule is that tadalafil must never be combined with nitrate heart medicines, because together they can drop blood pressure to dangerous levels [1].

Two ways to take it: as-needed vs. daily

Tadalafil is the only common ED pill sold in two distinct regimens, and the choice changes how you use it. As-needed dosing starts at 10 mg taken before anticipated sex, then moves up to 20 mg or down to 5 mg depending on how well it works and how you tolerate it [1]. Because a single dose lasts so long, many men take it hours ahead of time rather than timing it to the minute.

Daily dosing uses a smaller tablet, 2.5 mg to start and often raised to 5 mg, taken at the same time every day without regard to sexual activity [1]. The trade-off is steady readiness in exchange for taking a pill you may not use each day. In a pooled analysis of 1,913 men across six randomized, placebo-controlled trials, both the 2.5 mg and 5 mg daily doses significantly improved erectile function scores compared with placebo [2]. The 5 mg daily strength is also the version approved to treat the urinary symptoms of an enlarged prostate, so men with both conditions sometimes prefer it [1].

Onset, duration, and the 36-hour reputation

Tadalafil's long window is its signature. The FDA label states it improves erectile function versus placebo up to 36 hours after a dose, far longer than the roughly 4-to-6-hour window of standard sildenafil [1]. That is a duration of opportunity, not a 36-hour erection: the drug simply makes an erection easier to achieve in response to arousal across that period.

Onset is not instant. Most men see an effect within about an hour, and a heavy, high-fat meal matters far less for tadalafil than it does for sildenafil, so you do not have to take it on an empty stomach [1]. For daily users, the drug builds to a steady level over about five days, after which there is no on-off timing to think about at all. Sexual stimulation is still required for any of these regimens to work; the pill enables an erection but does not create one on its own.

The common side effects, and the back ache that is distinctive to tadalafil

The frequent side effects are the ones you would expect from a drug that relaxes blood vessels. At the 20 mg as-needed dose, the FDA label reports headache in about 15% of men, dyspepsia (indigestion) in about 10%, back pain in about 6%, and flushing, nasal congestion, muscle ache (myalgia), and limb pain each in about 3% [1]. These are usually mild, appear early, and often fade with continued use or a lower dose.

Back and muscle ache stand out because they are reported more with tadalafil than with other PDE5 inhibitors. The leading pharmacologic explanation is that tadalafil, unlike sildenafil, also inhibits an enzyme called PDE11 found in skeletal muscle, though this link remains a hypothesis rather than a proven cause. Practically, the ache typically starts 12 to 24 hours after a dose, sits in the lower back or thighs, and responds to ordinary pain relievers like acetaminophen. If it is severe or persistent, that is worth a conversation about switching pills.

The rare but serious risks

A handful of reactions are uncommon but need immediate action. Priapism, an erection lasting more than four hours, is a medical emergency; the FDA label directs anyone in that situation to seek emergency care, because prolonged erections can permanently damage penile tissue [1]. Do not wait it out.

PDE5 inhibitors also carry rare reports of sudden vision loss in one or both eyes, linked to a condition called non-arteritic anterior ischemic optic neuropathy (NAION), and separate rare reports of sudden decrease or loss of hearing, sometimes with ringing or dizziness [1]. In both cases the label instruction is the same: stop the drug and get prompt medical attention. These events are rare, and whether the drug causes them or simply coincides with underlying vascular risk is not fully settled, but the safe response does not depend on that answer.

The combination that can be dangerous: nitrates

The single hard contraindication is nitrates. Giving tadalafil to anyone taking any form of organic nitrate is contraindicated because tadalafil potentiates the blood-pressure-lowering effect of nitrates, and the combination can cause a sudden, severe, potentially fatal drop in blood pressure [1]. Nitrates include nitroglycerin tablets, patches, and sprays, isosorbide mononitrate and dinitrate, and the recreational inhalants known as poppers (amyl or butyl nitrite).

This matters most in an emergency. If you develop chest pain after taking tadalafil, tell the treating clinician you have taken it, because the usual nitrate given for cardiac chest pain would be unsafe while the drug is active, and with tadalafil that window stretches across roughly a day and a half. Men who use nitrates regularly for heart disease are not candidates for tadalafil at all.

Alpha-blockers, alcohol, and other interactions

Alpha-blockers, prescribed for high blood pressure or an enlarged prostate, are the next caution. Both alpha-blockers and tadalafil lower blood pressure, so combining them can cause dizziness or fainting from a drop on standing; the label advises stabilizing the alpha-blocker dose first and starting tadalafil at the lowest dose [1]. Certain other blood-pressure drugs and, importantly, the guanylate cyclase stimulator riociguat should also be avoided with PDE5 inhibitors.

Alcohol adds to the same effect. Both alcohol and tadalafil act as mild vasodilators, and taken together, especially in larger amounts, they can amplify a drop in blood pressure and its dizziness [1]. A drink or two is generally tolerated; heavy drinking with the drug is where problems appear. Finally, strong inhibitors of the liver enzyme CYP3A4, such as certain antifungals and HIV protease inhibitors, raise tadalafil levels and call for dose limits, which is a reason to give any prescriber your full medication list.

Choosing a dose, and when tadalafil is not right for you

Dose selection is individual. A reasonable starting point for as-needed use is 10 mg, moving to 20 mg if 10 is not enough or down to 5 mg if side effects bother you [1]. If you have sex more than about twice a week or dislike planning around a pill, the 2.5-to-5 mg daily route trades a bit of peak strength for constant readiness and, for some men, milder side effects [1][2]. Kidney or liver impairment lowers the ceiling on both regimens, so those doses are set by a clinician.

Tadalafil is not the answer for everyone. It is inappropriate for men on nitrates, for those whom a doctor has advised against sexual activity because of heart disease, and for anyone with certain rare eye conditions. ED is also frequently an early signal of cardiovascular or hormonal problems, so a new case deserves a real evaluation rather than just a prescription. Telehealth services and in-person clinicians both prescribe tadalafil after a health questionnaire or visit; the drug itself is identical regardless of who writes the prescription, so compare on price, medical oversight, and whether the questions actually screen for the nitrate and heart-risk issues above.