How they work
During arousal, nerve signals trigger the release of nitric oxide in the erectile tissue. This relaxes smooth muscle, increases blood flow, and produces an erection. An enzyme called PDE5 naturally limits this process by breaking down the key signalling molecule. PDE5 inhibitors block that enzyme, allowing the response to develop more fully.
They amplify what is already happening — they do not switch anything on independently. This is why context and genuine arousal matter as much as the medication itself.
The four options in the UK
| Medication | Duration | Onset | Food |
|---|---|---|---|
| Sildenafil (Viagra) | 4–5 hours | 30–60 min | Delayed by heavy meals |
| Tadalafil (Cialis) | Up to 36 hours | 30–60 min | Not affected |
| Vardenafil (Levitra) | 4–5 hours | 25–60 min | Delayed by heavy meals |
| Avanafil (Spedra) | 4–6 hours | 15–30 min | Not affected |
Tadalafil is also available as a low daily dose (2.5–5 mg), which removes the need to time each occasion. There is no single best option — the right choice depends on your lifestyle and how you respond individually.
Using them correctly
- Time it properly. Take the tablet at the interval indicated — typically 30–60 minutes before activity. Do not take it and immediately attempt sex.
- Be genuinely aroused. Sitting and waiting does not work — and usually makes things worse by increasing self-monitoring.
- Give it several tries. It typically takes four to eight properly-conducted attempts to know whether a medication at the right dose is working for you. Do not draw conclusions from a single occasion, especially under pressure.
- Alcohol. Moderate alcohol is fine. Heavy drinking impairs the erectile response independently and may amplify blood pressure effects.
Side effects
- Headache and facial flushing — common, mild, usually short-lived.
- Nasal congestion or indigestion.
- Tadalafil: back or muscle aches in some men, resolving within 1–2 days.
- Sildenafil at higher doses: a transient bluish tint to vision (rare).
Side effects tend to reduce with repeated use. If they persist, switching to a different agent usually resolves them.
Are these medications safe for the heart?
For most men — yes. The concern about cardiac risk comes from two things that are often conflated.
The nitrate interaction was identified when sildenafil was first approved in 1998. Some men who combined it with nitrate medication for angina (GTN spray or tablets) experienced severe drops in blood pressure. This is a pharmacological interaction — both drugs amplify the same pathway, and the combined effect can be dangerous. It is not evidence that PDE5 inhibitors damage the heart. Remove the nitrate, and this specific risk disappears entirely.
Heart failure is the other area of concern. The relevant caution here is not the medication — it is the physical exertion of sex itself (roughly equivalent to climbing two flights of stairs at a moderate pace). Men with severe, unstable heart failure may not be fit for that level of activity, regardless of what medication they take. Men with mild to moderate, stable heart failure who can tolerate moderate exertion can generally use PDE5 inhibitors safely. The question is exercise capacity, not drug toxicity.
If you have a prolonged erection
An erection lasting more than four hours (priapism) is a rare but serious complication requiring emergency medical care. Do not wait. Go to A&E immediately.