Sildenafil and tadalafil are both PDE5 inhibitors, they work by blocking the enzyme phosphodiesterase type 5, which breaks down cyclic GMP in penile smooth muscle. By preventing this breakdown, both drugs prolong the vasodilatory effect of nitric oxide and help maintain an erection. They are equally effective at their respective recommended doses. The meaningful differences are pharmacokinetic: how long they last and how food and timing affect their action.

Duration of Action

Sildenafil begins working in 30-60 minutes and lasts approximately 4-6 hours. For most men, the therapeutic window requires planning intercourse within a couple of hours of taking the dose.

Tadalafil has a half-life of approximately 17.5 hours and remains effective for up to 36 hours, earning it the informal name “the weekend pill.” This extended duration removes the timing pressure associated with sildenafil and allows for more spontaneous activity within the dosing window.

For men who prefer not to plan around medication timing, tadalafil has a practical advantage. For men who want medication active for a discrete window without it persisting into the next day, sildenafil’s shorter duration may be preferred.

Food Interactions

High-fat meals delay sildenafil absorption. A fatty meal taken 30-60 minutes before sildenafil can delay peak plasma concentration by 60 minutes and reduce the peak level by 29%. Taking sildenafil on an empty stomach or with a low-fat meal produces the most reliable and fastest onset.

Tadalafil is not meaningfully affected by food. It can be taken with or without food, and the timing relative to meals does not affect absorption. This adds to its practical convenience.

Daily vs. On-Demand Dosing

Tadalafil is the only PDE5 inhibitor approved for daily use at a lower dose (2.5-5 mg daily versus 10-20 mg on-demand). Daily low-dose tadalafil maintains constant blood levels, eliminating the need to time doses before sexual activity. It is also approved for benign prostatic hyperplasia (BPH) at the 5 mg daily dose, making it a dual-purpose option for men with both conditions.

A meta-analysis comparing on-demand and daily tadalafil found that daily dosing improved erectile function scores comparably to on-demand dosing, with the additional benefit of treatment effect on urinary symptoms in men with BPH.

Side Effects

Both drugs share a class-effect side effect profile:

  • Headache (most common, occurring in approximately 10-15% of users)
  • Facial flushing
  • Nasal congestion
  • Dyspepsia

Sildenafil-specific: Visual disturbances, blue-tinged vision, sensitivity to light, blurred vision, occur in approximately 3% of users. These result from mild inhibition of PDE6 in the retina and are dose-dependent and transient. Men who use nitrate medications for heart disease cannot take sildenafil due to dangerous blood pressure drops.

Tadalafil-specific: Back pain and myalgia (muscle aches) occur in approximately 5% of tadalafil users and are not typically seen with sildenafil. The mechanism is inhibition of PDE11, which is expressed in muscle. These effects are usually mild and transient.

Both are contraindicated with nitrates (nitroglycerin, isosorbide) due to synergistic blood pressure reduction. Caution is warranted with alpha-blockers (commonly prescribed for BPH or hypertension), which can also reduce blood pressure.

Efficacy Comparison

Multiple head-to-head trials and meta-analyses have compared sildenafil and tadalafil. A 2013 meta-analysis in the Journal of Sexual Medicine analyzed data from 82 trials and found that both drugs improved IIEF (International Index of Erectile Function) scores by 6-7 points on average versus placebo, with no statistically meaningful difference in efficacy between agents.

Tadalafil patients in preference studies tend to rate spontaneity and convenience higher, which may translate to better adherence over time. Whether this produces better real-world outcomes is harder to quantify.

Generic Availability and Cost

Both drugs are available as generics. Generic sildenafil has been available since 2017 and costs approximately $15-50 for a 30-tablet supply through discount pharmacies (GoodRx pricing). Generic tadalafil became available around the same time at comparable pricing.

Brand-name Viagra and Cialis are substantially more expensive and are rarely prescribed given the equivalent efficacy and availability of generics.

Which to Choose

No clinical guideline recommends one over the other for the general population. The choice depends on frequency of sexual activity, preference for spontaneity versus planned timing, tolerance for individual side effects, and whether BPH symptoms are present.

For a broader look at what causes erectile dysfunction and what the full range of treatments involves, see Erectile Dysfunction: Causes, Risk Factors, and What the Evidence Shows.