Tadalafil and Sildenafil Side Effects: What's Common and What Isn't

Sildenafil and tadalafil belong to a class called PDE5 inhibitors. They do not create arousal. Rather, think of them as amplifiers: they improve erections after arousal and erotic stimulation, but do not cause erections without stimulation. When you are aroused, your body releases nitric oxide, which triggers a chemical signal that relaxes the arteries feeding the penis. An enzyme called PDE5 breaks that signal down. These drugs slow the breakdown, so the signal lasts longer and the response is stronger.

That is why they only work alongside real arousal, and it is also why the side effects look the way they do. The enzyme these drugs block is not confined to the penis. Blocking it relaxes blood vessels elsewhere in the body, which may produce headache, facial flushing, a stuffy nose, or even back pain. Those three effects are the same mechanism as the intended one, showing up where you did not want it.

From the Cialis trials of as-needed dosing across eight placebo-controlled studies:

There is a dose relationship. Headache triples from placebo to 20 mg, and indigestion goes up tenfold. If side effects are the problem, a lower dose is usually the first thing to try rather than a different drug.

Overall, 3.1% of men taking tadalafil 10 or 20 mg stopped treatment because of side effects, against 1.4% on placebo. That means roughly 97% stayed on it. So, men may have side effects, but it is uncommon for the side effects to make a man stop treatment.

Daily dosing at 2.5 mg and 5 mg produces a milder profile. In the 12-week trials, headache was reported by 3% at 2.5 mg and 6% at 5 mg, against 5% on placebo. Indigestion ran 4 and 5% against 2%. At those doses the numbers sit close to placebo for most reactions.

From the Viagra trials, at the doses most commonly prescribed:

Sildenafil produces more headache and flushing than tadalafil at equivalent positions in the dose range, and it has one effect tadalafil does not: a temporary blue tinge to vision or increased light sensitivity, reported by 11% of men at 100 mg. Two things drive the difference. Sildenafil reaches a higher peak concentration faster, so the vasodilation causing headache and flushing arrives more sharply. And sildenafil weakly blocks a second enzyme, PDE6, found in the retina, which tadalafil does not do to any meaningful degree. That happens because sildenafil also weakly blocks a related enzyme in the retina. It is transient and it resolves as the drug clears. It is not the same as the vision loss mentioned below.

Of the common side effects, this one is probably the most surprising.

Back pain and muscle aches occur in roughly 3 to 6% of men on tadalafil. In the clinical pharmacology trials it typically appeared 12 to 24 hours after the dose and resolved within 48 hours. It was described as a diffuse ache across the lower back, buttocks or thighs, usually mild or moderate, and it got worse lying down. The cause is not settled. The leading explanation is that tadalafil weakly blocks a related enzyme, PDE11, present in skeletal muscle, and that its long half-life gives that effect time to show up. It is NOT a sign of injury. Severe back pain was reported in fewer than 5% of those cases, and about 0.5% of all men on as-needed tadalafil stopped treatment because of it.

It resolves on its own. Simple painkillers help. If it recurs every time, that is a reason to try a lower dose or a different drug in the class.

Take the lowest dose that works. Most side effects here are dose-dependent, so this is the single most effective lever.

Give it a few attempts. A single disappointing night usually reflects timing, a heavy meal, alcohol or too little arousal rather than the drug failing. See our article on when your ED medication stops working.

Consider which drug. Tadalafil produces less headache and flushing than sildenafil at comparable doses, but it stays in your system far longer, so a side effect you do get lasts longer. Sildenafil is harder on the head and clears faster. Neither is better; they fail differently.

Switching within the class is reasonable. These two drugs have somewhat different side effect profiles, and men who tolerate one poorly often do fine on the other.

These side effects are uncommon. They are also the reason to read this section before you need to.

An erection lasting more than four hours: This is called priapism and it is a medical emergency. This is an erection that is not associated with arousal. Blood trapped in the penis loses oxygen, and after several hours that can cause permanent damage to the tissue responsible for erections. Do not wait to see if it resolves - climax, cold showers, exertion, etc. - do not make it go away. Go to an emergency department.

Sudden loss of vision in one or both eyes: This has been reported rarely with PDE5 inhibitors and may represent a condition affecting the blood supply to the optic nerve. Stop the medication and seek immediate attention.

Sudden decrease or loss of hearing, sometimes with ringing in the ears or dizziness: Same instruction: stop and seek care.

Chest pain during or after sex: Stop, seek emergency care, and tell the treating clinician you have taken a PDE5 inhibitor and when. That last detail matters enormously, because it determines whether they can safely give you nitrates.

Nitrates are an absolute contraindication. That includes nitroglycerin in any form, isosorbide, and the recreational nitrites sold as poppers. The combination can cause a severe drop in blood pressure. We cover this in detail in our article on nitrates.

Men taking alpha-blockers, men with significant cardiovascular disease, uncontrolled blood pressure, recent heart attack or stroke, or serious liver or kidney impairment all need individual assessment.

The common side effects of these medications are headache, flushing, indigestion, a blocked nose and back pain, and they are the same mechanism as the intended effect appearing where you did not want it. They are dose-dependent, which makes the lowest effective dose the first lever to pull, and daily low-dose tadalafil produces rates close to placebo. Around 97% of men in the trials kept taking the medication. Four reactions require immediate care: an erection past four hours, sudden vision loss, sudden hearing loss, and chest pain during sex.