Three levels of interaction
Forbidden: nitrates and poppers — with any PDE5 inhibitor they can crash blood pressure. Doctor first: alpha-blockers, strong antifungals, HIV protease inhibitors, riociguat, significant heart or liver disease. Practical: alcohol in moderation, fatty meals (they delay sildenafil), grapefruit juice (it raises the level of the tablet).
Why these medicines interact at all
Almost every interaction in this class comes from one of two mechanisms, and knowing which makes the rules easy to remember.
The first is shared pathway. PDE5 inhibitors work inside the nitric-oxide signalling that widens blood vessels. Nitrates push the same pathway from the other end. Combine them and the effect multiplies rather than adds — which is why this one rule is absolute rather than a matter of dose or timing.
The second is shared metabolism. These tablets are broken down by the liver enzyme CYP3A4. Anything that inhibits it — ketoconazole, ritonavir, clarithromycin, grapefruit juice — leaves more drug in your system for longer, which shows up as stronger side effects. Anything that induces it, such as rifampicin or St John's wort, clears the drug faster and can make it seem not to work.
The interaction people miss
Some medicines do not clash with an ED tablet; they help cause the problem. Certain beta-blockers, thiazide diuretics, SSRIs and antipsychotics are recognised contributors to erectile difficulty. If your symptoms began within weeks of starting something new, that timing is worth more than any search result — take it back to the prescriber, who can often switch you within the same class.
Checking properly
Write down everything you take, including supplements and anything herbal, and ask a pharmacist. It is free, it takes a minute, and it is the single highest-value minute in this whole section. The guides below work through alcohol and the medicine-by-medicine map in detail; product-specific cautions live on generic Viagra and generic Cialis.
What counts as a medicine here
People check prescriptions and forget everything else, which is where most avoidable interactions come from. For this purpose a medicine is anything you swallow, inhale, inject or apply for an effect: prescriptions, pharmacy medicines, supplements, herbal preparations, and recreational drugs.
Two of those deserve naming. St John's wort is bought off a shelf and interacts with a long list of drugs including this class. Poppers are recreational, rarely mentioned to a doctor, and fall squarely under the nitrate rule that makes them genuinely dangerous with any ED tablet. A pharmacist cannot warn you about something you do not mention.
