Tadalafil
Cialis
Tadalafil (Cialis) is a hormonal & sexual research compound. PDE5 inhibitor — blocks the enzyme that breaks down cGMP, so nitric oxide signaling produces a longer, stronger vasodilatory response. The effect is systemic, not just genital, which is why the low daily dose gets used for reasons that have nothing to do with erections.
Tadalafil quick facts
| Reported research dose | 2.5–5mg daily, or 10–20mg as needed |
| Route | Oral |
| Frequency | 1x · Daily or PRN |
| Half-life | ~17.5 hours — by far the longest of the PDE5 inhibitors, which is what makes daily dosing practical. |
| Forms | Oral |
| Evidence level | Large RCTs for ED and BPH; smaller trials for endothelial function |
The daily low dose is the interesting one. Beyond ED it has approved use in BPH, real trial data on endothelial function, and it's widely used on cycle to offset the blood-pressure and vascular cost of AAS. ⚠️ Absolutely contraindicated with nitrates — the combination can drop blood pressure catastrophically. Check BP and hematocrit.
How Tadalafil works
PDE5 inhibitor — blocks the enzyme that breaks down cGMP, so nitric oxide signaling produces a longer, stronger vasodilatory response. The effect is systemic, not just genital, which is why the low daily dose gets used for reasons that have nothing to do with erections.
Proposed benefits
Researched for libido, hormonal signaling and reproductive / sexual function.
Where to get Tadalafil
Buy Tadalafil at AlgoRx →The evidence for Tadalafil
Graded by what exists behind each claim.
✅ Clinically validated
- Approved for erectile dysfunction, BPH and pulmonary hypertension with large randomized datasets. Daily low-dose tadalafil has trial support for BPH symptoms independent of erectile function.
- Growing research interest in vascular and endothelial endpoints beyond the licensed uses.
📊 Correlative data
- Widely used daily rather than on demand, which the pharmacology supports. Reported experience includes back and limb ache in a minority — a recognized class effect attributed to PDE11 cross-reactivity.
🧪 Theoretical / extrapolated
- A PDE5 inhibitor with a ~17.5-hour half-life against sildenafil's four, which is the entire practical difference and why daily dosing is viable.
- The long half-life plus PDE11 cross-reactivity predicts both the 'always-ready' effect people choose it for and the muscular aching some get. Same absolute nitrate contraindication.
How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →
What Tadalafil actually does
Tadalafil does not create a signal. It prevents one from being switched off — and every clinically important property of the drug, including the two that make it different from the rest of its class, follows from which enzymes it stops.
The pathway, in order. A nerve or endothelial cell releases nitric oxide. NO activates soluble guanylate cyclase, which makes cyclic GMP from GTP. cGMP activates protein kinase G, which lowers intracellular calcium in vascular smooth muscle and relaxes it. Phosphodiesterase type 5 is the enzyme that hydrolyzes cGMP back to inactive GMP. Inhibit PDE5 and cGMP accumulates wherever it was already being made. The drug is permissive, not causative: with no NO release there is no cGMP to preserve, which is the mechanistic reason a PDE5 inhibitor does nothing without a stimulus, and the reason it is not an aphrodisiac Wallis 1999.
Now the selectivity table, which is the most informative thing on the label and appears on almost no consumer page. Tadalafil is more than 10,000-fold more potent for PDE5 than for PDE1, PDE2, PDE3, PDE4 and PDE7; more than 9,000-fold over PDE8, PDE9 and PDE10; 700-fold over PDE6; and only 14-fold over PDE11A1 and 40-fold over PDE11A4 Cialis label 2026.
Read the two ends of that table against the side-effect list. PDE6 is the retinal phosphodiesterase of the phototransduction cascade; sildenafil is closer to PDE6 than tadalafil is, which is why blue-tinged vision is a sildenafil story Wallis 1999. PDE11A is expressed in skeletal muscle, prostate and testis — and tadalafil's separation from it is only 40-fold, the narrowest margin the molecule has to anything. Back pain occurs in about 6% of users and myalgia in 3–4% Cialis label 2026, and those two entries are the ones that distinguish tadalafil's adverse-effect profile from every other PDE5 inhibitor. A 40-fold margin to an enzyme expressed in muscle, and a muscle side effect nobody else in the class has, is the single most quotable fact about this drug — and it is a strong association from the label's own numbers, not a proven causal chain.
The half-life is a separate property and it is the commercial one. Terminal elimination is about 17.5 hours, with a median time to peak of 1.0 hour, roughly 94% plasma protein binding, predominant metabolism by CYP3A4, and no meaningful food effect Cialis label 2026. Seventeen and a half hours is what makes once-daily dosing sensible and what earned the drug its nickname; sildenafil's short half-life makes it an event drug Eardley 2002.
Cell, rodent, human — and where it stops
Step one, enzymology: settled, and quantified on the label itself. The isoform selectivity ratios above are regulatory data Cialis label 2026, and the class pharmacology is reviewed against the current medicinal chemistry ElHady 2023.
Step two, the second indication, which is where the daily dose came from. Roehrborn 2008 was a dose-finding study of tadalafil administered once daily for lower urinary tract symptoms secondary to benign prostatic hyperplasia. That is a different organ and a different endpoint from erectile function — the read-out is a symptom score, not an event — and it is the reason a 5 mg daily tablet exists at all.
Step three, the 5 mg daily dose examined on its own. Cui 2021 is an evidence-based analysis of 12-week monotherapy with once-daily 5 mg tadalafil for the lower urinary tract symptoms of benign prostatic hyperplasia. Twelve weeks is the honest timescale: the urinary effect is not immediate the way the erectile effect is, because it depends on chronic smooth-muscle and possibly endothelial change in the prostate and bladder neck rather than on a single episode of cGMP accumulation.
Step four, the extrapolation that is being tested right now. PDE5 inhibitors have molecular effects on male reproductive tissue that are being examined for fertility management Kaltsas 2025 — a use that follows from PDE5 and PDE11 expression in the testis rather than from any vascular argument. This is a live research question, clearly labeled as one, and it is exactly the class of extrapolation this site exists to state without dressing it up as established.
Where the chain breaks. (1) The urinary trials use symptom scores, which are subjective instruments with large placebo responses; the 12-week analysis Cui 2021 inherits that limitation. (2) The PDE11 argument for back pain is an association between a selectivity number and an adverse-event rate on the same label Cialis label 2026; nobody has knocked out PDE11 in a human and watched the back pain disappear. (3) The endothelial and reproductive extrapolations Kaltsas 2025 are mechanism-led and their clinical trials are mostly small. (4) Comparative claims between PDE5 inhibitors mostly rest on separate trials against placebo rather than on head-to-head randomization Goldstein 1998.
What would have to be true, and how you would know it was not
Three predictions. The second is the one that would falsify the PDE11 story, and the third is the one that matters for safety.
1. If the drug is working through cGMP, the effect is stimulus-dependent and an instrument can show it. The read-outs exist and are validated: the IIEF questionnaire for erectile function, and for the urinary indication the IPSS symptom score with uroflow and post-void residual Roehrborn 2008 Cui 2021. Score them before starting and at 12 weeks, because the urinary effect takes that long and a four-week judgment on the daily dose is a judgment made too early.
2. The falsification test for the PDE11 argument is your own back. The claim is that back pain and myalgia — 6% and 3–4% on the label Cialis label 2026 — come from the narrow 40-fold margin over PDE11A4 in skeletal muscle. That predicts a dose-dependent symptom appearing within a day or two of dosing and resolving as the drug clears over roughly three half-lives, about two days. If back pain appears on tadalafil, is dose-related, and does not appear on a short-acting PDE5 inhibitor with a wider PDE11 margin, the mechanism is behaving as advertised. If it is constant and unrelated to dosing, it is not the drug.
3. The safety prediction, and it is a hard stop rather than a trend. Sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing, are both labeled events for which the instruction is to stop the drug and seek medical attention immediately Cialis label 2026. Neither is a side effect to tolerate. Separately, a CMP is worth having at baseline in anyone with kidney or liver impairment, because both raise exposure to a drug cleared by CYP3A4, and a lipid panel plus blood pressure belongs in the same visit — erectile dysfunction in a middle-aged man is a vascular sign before it is a sexual one, and treating the symptom without looking at the vasculature is the most common mistake made with this drug.
What nobody has tested yet
Four things nobody has established.
Nobody has proved the PDE11 hypothesis. The 40-fold margin Cialis label 2026 and the muscle symptoms sit on the same page and have never been connected by an experiment. A study measuring PDE11A4 occupancy in human skeletal muscle against symptom reports at different doses would settle it, and would also say whether the myalgia is avoidable by dose.
Nobody has run the long endothelial trial. Chronic daily PDE5 inhibition raises cGMP tone continuously, and endothelial function is measurable by flow-mediated dilation. A randomized trial of 5 mg daily against placebo for a year with FMD and pulse wave velocity as endpoints would test the vascular claim directly. Nobody has published one — the same gap this site keeps naming across compound after compound.
Nobody has stratified response by CYP3A4 activity. Tadalafil is predominantly a CYP3A4 substrate Cialis label 2026. CYP3A4 activity varies several-fold between people and is modified by common drugs and foods. Responders, non-responders and people with disproportionate side effects have never been separated by measured CYP3A4 activity, and that is a straightforward study.
Nobody has tested the fertility extrapolation properly. The molecular groundwork exists Kaltsas 2025. The randomized trial with semen parameters and pregnancy as endpoints, in men without erectile dysfunction, does not.
Tadalafil — its own safety story, not its class's
Tadalafil is a prescription medicine with a full label, so the generic caution block above is the wrong instrument. Here is what is specific and what is absolute.
Nitrates are an absolute contraindication and the reason is arithmetic, not caution. Organic nitrates donate nitric oxide; PDE5 inhibition prevents the resulting cGMP from being cleared. The two act at consecutive steps of the same pathway, so the effects multiply rather than add, and the result is profound hypotension. The label states plainly that administration to a patient using any form of organic nitrate is contraindicated Cialis label 2026 — and the potentiation was documented for this class before any of these drugs were widely used Webb 2000. Because tadalafil's half-life is 17.5 hours, the window in which this matters is roughly two days after a dose, not a few hours, and that is the part people get wrong. It matters most in exactly the situation nobody plans for: chest pain, an ambulance, and a nitrate given by somebody who was not told.
Guanylate cyclase stimulators are the second absolute block. Riociguat activates the enzyme upstream of the one tadalafil protects. The label says do not use them together Cialis label 2026. Same pathway, same multiplication.
Alpha-blockers are the near-miss. Many men taking tadalafil for urinary symptoms are also on an alpha-blocker for the same prostate. Both lower blood pressure by different routes. This is a sequencing and dose conversation with a prescriber rather than a prohibition, and it is a real one.
The labeled adverse-event rates, so the trade is visible: headache 15% at the 20 mg as-needed dose, dyspepsia 10%, back pain 6%, myalgia 3–4%, and nasal congestion, flushing and pain in a limb about 3% each Cialis label 2026.
Two events that are stop-immediately rather than tolerate. Sudden vision loss, which is the presentation of non-arteritic anterior ischemic optic neuropathy, and sudden hearing loss. Both are on the label with the same instruction: stop and seek medical attention Cialis label 2026.
What this page will not do. Recommend a dose, or treat erectile dysfunction as a standalone complaint. Erectile dysfunction is frequently the first symptom of endothelial disease, and a drug that makes the symptom go away without anybody checking the blood pressure, the lipids or the glucose has hidden a warning rather than answered one.
Sources read for this page
- U.S. Food and Drug Administration. CIALIS (tadalafil) tablets, for oral use — full prescribing information, including phosphodiesterase selectivity ratios and the nitrate contraindication. DailyMed, U.S. National Library of Medicine; label revised 2026
- Roehrborn CG, et al. Tadalafil administered once daily for lower urinary tract symptoms secondary to benign prostatic hyperplasia: a dose finding study. Journal of Urology 2008 · PMID 18722631
- Cui J, et al. Efficacy and Safety of 12-week Monotherapy With Once Daily 5 mg Tadalafil for Lower Urinary Tract Symptoms of Benign Prostatic Hyperplasia: Evidence-based Analysis. Frontiers in Medicine 2021 · PMID 34712682
- ElHady AK, et al. Advancements in Phosphodiesterase 5 Inhibitors: Unveiling Present and Future Perspectives. Pharmaceuticals (Basel) 2023 · PMID 37765073
- Kaltsas A, et al. Phosphodiesterase Type 5 Inhibitors in Male Reproduction: Molecular Mechanisms and Clinical Implications for Fertility Management. Cells 2025 · PMID 39851548
Tadalafil — safety, predicted from mechanism
Predicted from mechanism, not from a human safety trial. How that reasoning works →
What the mechanism predicts
Derived from the molecule, not a trial.
- These inhibit phosphodiesterase type 5, so cGMP persists and smooth muscle relaxes — vasodilation. The predicted problems are all the same effect in the wrong place: headache, flushing, nasal congestion and a fall in blood pressure.
- Tadalafil's long half-life (~17.5 h) means its effects — good and bad — persist far longer than sildenafil's. That is why daily low-dose tadalafil exists and why a mistake with it lasts longer.
What has actually been reported
- Extremely well characterized — these are among the most-prescribed drugs in the world. Headache, flushing, dyspepsia and nasal congestion are common and dose-dependent.
- Rare but serious: sudden hearing loss, and non-arteritic anterior ischemic optic neuropathy (NAION) — sudden painless vision loss in one eye. Both are reasons to stop immediately and seek care.
- Priapism — an erection lasting over four hours — is a medical emergency, not something to wait out. Delay causes permanent damage.
How to reduce the risk
Same mechanism as the prediction.
- Start at the low end. The dose-response for side effects is steeper than for benefit.
- Alcohol and these both lower blood pressure; the combination is the usual cause of feeling awful on a normal dose.
What it does to your bloodwork
A fact about the assay.
- Nothing routine. Blood pressure is the measurement that matters and it is not a lab.
Don't run this if
- You take any nitrate — including recreational 'poppers' (amyl nitrite). The combination causes a catastrophic drop in blood pressure and is the one genuinely lethal interaction in this class.
- You take an alpha-blocker, without a prescriber managing the timing.
- You have significant cardiovascular disease or have been advised against sexual activity for cardiac reasons.
Not medical advice. If you take prescription medication or have a diagnosed condition, check this with a pharmacist or doctor.
Tadalafil — interference & stacking
Predicted from mechanism, not from an interaction study. How mechanism-predicted claims are made →
What Tadalafil moves on your bloodwork
Expected direction, not a measured one.
- Complete Blood Count (CBC) with Differential — ↑ expected to rise
Hematocrit and hemoglobin rise — androgens stimulate erythropoiesis. This is the most reliably predictable movement of any compound in the Vault.
What to do: This is the number that decides whether you keep going. Baseline and every 3 months. Dehydration on the draw day inflates it, so hydrate normally or you will chase a false reading. - Total Testosterone — ↑ expected to rise
Expected. Trough vs peak matters enormously — the same protocol reads completely differently depending on when you drew.
What to do: Draw at the same point in the cycle every time or the trend is noise. - LH & FSH — ↓ expected to fall
Suppressed by negative feedback. This is the mechanism, not a side effect — and it is why exogenous androgen shuts down your own production.
What to do: Relevant if fertility matters to you. Worth knowing before, not after. - Estradiol, Sensitive (LC/MS-MS) — ↑ expected to rise
Aromatization converts a fraction to estradiol, and it rises with the androgen. Use the sensitive (LC-MS/MS) assay — the standard immunoassay is unreliable in men and produces numbers people then medicate.
What to do: If you are reading estradiol in a man, the assay choice matters more than the result. - SHBG (Sex Hormone-Binding Globulin) — ↓ expected to fall
Falls with androgen exposure, which raises the free fraction — so free testosterone can climb faster than total.
What to do: Read total and SHBG together; total alone understates what changed. - Lipid Panel (Cholesterol, HDL, LDL, Triglycerides) — ↓ expected to worsen
HDL falls, sometimes markedly. Oral 17-alpha-alkylated compounds do this far more aggressively than injectable esters.
What to do: Baseline and 12 weeks. ApoB is the better long-term read than LDL-C. - PSA (Total + Free + % Free) — ↑ expected to rise
Androgens can raise PSA modestly. It does not create prostate cancer that wasn't there, but it can unmask it.
What to do: Baseline before starting matters — without it, a later number has nothing to be compared against.
- How to work up to it, and when not to
- When to take it, and why that window
- Cycle length
- Time off between cycles
- Fasted or fed, and when in the day
- Coach Cam's personal notes
- Which compounds push the same lever, and why the dose adds up faster than people count
- What blunts it — the stacks that waste your money
- What compounds the risk, so a side effect arrives sooner than any one of them suggests
- Coach Cam's read on running it alongside the rest of your protocol
Everything above is free and stays free. Skool is where it becomes a plan — Tadalafil in an order, with the rest of what you're running.
Unlock in Skool — $10/mo →Bloodwork to run alongside Tadalafil
Baseline first, then again at 8–12 weeks.
| Marker | What it’s watching for |
|---|---|
| Total Testosterone | The baseline you can't reconstruct later |
| Free Testosterone | The fraction that does anything — total alone misleads |
| SHBG (Sex Hormone-Binding Globulin) | Explains a normal total sitting on top of a low free |
| Estradiol, Sensitive (LC/MS-MS) | The other half of the ratio, and the source of most symptoms |
| LH & FSH | Separates a testicular problem from a pituitary one |
The “Low T? Rule Out the Reversible Causes First” panel covers these in one order — 11 markers, $211.50 with the discount applied.
Check results you already have → · All 103 markers A–Z
Tadalafil — frequently asked questions
What is Tadalafil?
Tadalafil (Cialis) is a hormonal & sexual research compound. PDE5 inhibitor — blocks the enzyme that breaks down cGMP, so nitric oxide signaling produces a longer, stronger vasodilatory response. The effect is systemic, not just genital, which is why the low daily dose gets used for reasons that have nothing to do with erections.
Is the full Tadalafil protocol on this page?
The reported research dose is on this page, along with how Tadalafil works and the evidence behind it. The protocol — how to work up to it, frequency, cycle length, time off, what not to stack it with and Coach Cam's notes — is inside Skool.
What is the half-life of Tadalafil?
Tadalafil has an approximate half-life of ~17.5 hours — by far the longest of the PDE5 inhibitors, which is what makes daily dosing practical., which is part of what determines how often it's dosed.
What's the evidence behind Tadalafil?
Current evidence level: Large RCTs for ED and BPH; smaller trials for endothelial function. Tadalafil is offered for research purposes only and is not an approved medicine.
Tadalafil inside a finished plan
One arm of 1 Protocol Blueprint, free to read in full.
What Tadalafil is used for
Tadalafil appears under 1 goal in the goal router.
Related Hormonal & Sexual compounds
Where this goes next
Tadalafil is the vascular arm of this plan. The page above is the free breakdown of one compound; the plan it belongs to — the dosing, the order to correct things in, the week-by-week schedule and what to retest — is a lesson inside Skool.