how long after official expiration date would Viagra still realistically work at least 80%  

Short answer: if your Viagra (sildenafil) tablets were stored well (cool, dry, in the original, tightly closed container), they usually don’t suddenly “stop working” at the printed date. Solid sildenafil is chemically pretty robust in the dry tablet form; stress-testing shows little degradation with heat/UV and high humidity over weeks to months, and oxidation problems mainly show up in solutions—not tablets. Thai Science+1
What that means in practice  
The labeled shelf life is typically ~2 years from manufacture; after that the maker no longer guarantees ≥90% potency. Healthline    
For intact tablets stored properly, available stability data suggest potency loss is usually slow. A conservative rule of thumb many pharmacists use for solid oral drugs is that they often remain close to full strength for at least 6–12 months past expiry, and quite possibly longer; hitting ≥80% potency is plausible well past the date if storage was ideal—though it’s not guaranteed lot-by-lot. (There isn’t a sildenafil-specific post-expiry potency curve published for consumer tablets.) ScienceDirect+1

This does not apply to liquids: the sildenafil oral suspension (Revatio) must be discarded 60 days after it’s mixed. FDA Access Data

 
Practical takeaways
If the tablets are only a few months past the date and were stored right, there’s a decent chance they’re still ≥80% potent. If they’re years past or saw heat/humidity (bathroom cabinet, car glovebox), expect reduced effect. ScienceDirect

Don’t use crumbly, discolored, or odorous tablets; get a fresh supply.
Safety first: don’t take sildenafil with nitrates (nitroglycerin/isosorbide), use caution with alpha-blockers, and check other conditions/meds with your clinician.
If you want, tell me how old the tablets are and how they were stored, and I’ll give you a more tailored risk/benefit read based on those specifics.
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