Quick answer
What is sildenafil and tadalafil used for?
Sildenafil (Viagra) and tadalafil (Cialis) improve erections by increasing blood flow when you are sexually aroused — they work for about 70 to 80% of men with physical ED. Sildenafil lasts 4 to 6 hours; tadalafil up to 36 hours or can be taken daily at low dose. Available on NHS prescription and from UK pharmacies. Never take with nitrates (GTN spray) or poppers.
PDE5 inhibitors — sildenafil and tadalafil
Sildenafil (Viagra) and tadalafil (Cialis) are PDE5 inhibitors — the first-line medical treatment for erectile dysfunction with a physical component. They increase blood flow to the penis when you are sexually aroused — they do not create desire or spontaneous erections without stimulation.
How they work
During arousal, nerves release nitric oxide, raising cGMP, which relaxes smooth muscle in penile arteries. PDE5 breaks down cGMP — inhibitors prolong the signal, improving rigidity and maintenance.
Require intact nerve pathways and arousal — will not work if testosterone is severely low (until corrected) or if purely psychogenic without any arousal.
Sildenafil (Viagra, generic)
| Aspect | Detail |
|---|---|
| Doses | 25, 50, 100mg |
| Onset | 30 to 60 minutes |
| Duration | 4 to 6 hours |
| Food | High-fat meal delays absorption — take on empty stomach for fastest effect |
| Frequency | Max once per 24 hours |
| NHS | Generic sildenafil on prescription — quantity limits vary by ICB |
Viagra Connect: 50mg pharmacy-only — pharmacist consultation — no prescription needed for eligible men.
Tadalafil (Cialis, generic)
| Aspect | Detail |
|---|---|
| On-demand doses | 10, 20mg |
| Daily doses | 2.5, 5mg |
| Onset | 30 minutes (daily dosing removes timing pressure) |
| Duration | Up to 36 hours (“weekend pill”) |
| Food | Minimal interaction |
| Daily 5mg | Also licensed for benign prostatic hyperplasia (BPH) urinary symptoms |
Advantage: Less planning; daily low dose suits frequent activity or combined ED/BPH.
Effectiveness
70 to 80% of men with physical ED achieve improved erections sufficient for intercourse.
Less effective when:
- diabetes with neuropathy
- post-radical prostatectomy (nerve damage)
- severe vascular disease
- low testosterone untreated
Try alternate PDE5 inhibitor if first fails — non-responders to sildenafil sometimes respond to tadalafil.
Getting them in the UK
- GP prescription — after ED assessment and cardiovascular review
- Pharmacy supply — Viagra Connect, some tadalafil brands — pharmacist checklist
- Private online clinics — must be regulated — check GPhC registration
Avoid: unregulated websites selling counterfeit tablets — may contain wrong dose or contaminants.
Side effects
Common:
- headache
- flushing
- dyspepsia
- nasal congestion
- back pain (tadalafil more than sildenafil)
- limb pain (tadalafil)
Visual: blue tinge, light sensitivity — rare; sudden vision loss (NAION) — extremely rare — stop and seek help
Hearing loss — rare — reported
Critical safety — nitrates and poppers
Absolute contraindication:
- GTN spray/tablets (angina)
- Isosorbide mononitrate/dinitrate
- Recreational poppers (amyl nitrite)
Combined effect causes profound hypotension — potentially fatal. If you have angina, discuss with cardiologist — ED treatment may still be possible with alternative angina management in stable disease.
Other interactions and cautions
- Alpha-blockers (tamsulosin, doxazosin) — hypotension risk — separate dosing times; start PDE5 inhibitor at lowest dose
- CYP3A4 inhibitors (ketoconazole, erythromycin) — increase PDE5 levels — lower dose sildenafil
- Recent stroke or MI — usually defer 6 months
- Unstable angina, severe heart failure — avoid
- Retinitis pigmentosa — contraindicated
- Anatomical deformity (Peyronie’s severe) — caution
Priapism — emergency
Erection lasting over 4 hours without sexual stimulation — phone 999 or attend A&E — tissue damage and permanent ED risk if untreated.
Cost comparison
Generic sildenafil — cheapest NHS option — pennies per tablet.
Tadalafil daily — higher cost but convenience premium.
Branded Viagra/Cialis — same drug, higher price — no clinical advantage over generic.
Lifestyle alongside tablets
PDE5 inhibitors work best combined with:
- smoking cessation
- weight loss
- blood pressure, diabetes, cholesterol control
- reduced alcohol
- pelvic floor exercises
- psychosexual therapy if performance anxiety contributes
Tablets treat symptoms — addressing underlying vascular health improves long-term outcomes and may reduce need for ongoing medication.
What if PDE5 inhibitors fail?
GP or urology referral for:
- vacuum erection device — NHS prescription possible
- intracavernosal alprostadil injections
- urethral alprostadil pellets
- testosterone if hypogonadism confirmed
- penile implants — last resort
ED is common and treatable — sildenafil and tadalafil help most men safely when prescribed appropriately.
Common questions
- How much sildenafil should I take?
- Standard starting dose 50mg taken 30 to 60 minutes before sex — adjust to 25mg if side effects or 100mg if insufficient response. Maximum one dose per 24 hours. Generic sildenafil is identical to Viagra — same active ingredient, lower cost.
- Can I take tadalafil every day?
- Yes — tadalafil 2.5mg or 5mg daily provides continuous effect — useful if you prefer spontaneity without timing doses. On-demand dosing is 10 to 20mg at least 30 minutes before sex, max once daily. Daily dosing suits men who have sex frequently or also have benign prostate enlargement (licensed indication at 5mg daily).
- Why did Viagra not work for me?
- Common reasons — insufficient dose, taking with heavy meal (sildenafil), lack of arousal, severe vascular disease, uncontrolled diabetes, low testosterone, or predominantly psychogenic ED. Try alternate PDE5 inhibitor, ensure arousal, or see GP for second-line options — vacuum device, injections, or referral.
- Are Viagra and Cialis available over the counter?
- Sildenafil 50mg (Viagra Connect) and tadalafil (some brands) are available from UK pharmacies after pharmacist consultation — no GP appointment needed for eligible men. Pharmacist checks safety — heart medicines, blood pressure, symptoms needing GP. NHS prescription often cheaper for regular use.
- What are common side effects of Viagra?
- Headache, facial flushing, indigestion, nasal congestion, dizziness, temporary blue-tinted vision (rare). Usually mild and short-lived. Chest pain during sex — stop and seek urgent help — could be cardiac event. Priapism — erection over 4 hours — emergency.
- Can I take Viagra with blood pressure tablets?
- Often yes — PDE5 inhibitors mildly lower blood pressure. Caution with alpha-blockers like tamsulosin — separate timing. Never with nitrates. Stable heart disease may allow use after cardiologist assessment — usually wait 6 months after heart attack. Unstable angina — contraindicated.