The interaction between cannabis and sildenafil (Viagra) is an increasingly relevant clinical question as cannabis use becomes more prevalent across the UK. Sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor licensed for erectile dysfunction and pulmonary arterial hypertension, is metabolised by hepatic cytochrome P450 enzymes. Cannabis contains tetrahydrocannabinol (THC) and cannabidiol (CBD), which interact with the endocannabinoid system and affect cardiovascular function. Whilst major drug interaction databases contain limited information on herbal cannabis and sildenafil interactions, both substances independently affect the cardiovascular system, raising theoretical concerns about concurrent use. Healthcare professionals should proactively enquire about all substance use, including cannabis, when prescribing sildenafil to ensure patient safety.

Does Cannabis Affect Sildenafil?

The interaction between cannabis and sildenafil (commonly known as Viagra) is an increasingly relevant clinical question, particularly as cannabis use becomes more prevalent in the UK. While major drug interaction databases contain limited information on interactions between herbal cannabis and sildenafil, this does not mean the combination is without potential concerns.

Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor licensed for the treatment of erectile dysfunction and pulmonary arterial hypertension. It works by enhancing the effects of nitric oxide, leading to smooth muscle relaxation and increased blood flow to specific areas of the body. The medication is metabolised primarily by the hepatic cytochrome P450 enzyme system, particularly CYP3A4 and CYP2C9.

Cannabis contains numerous active compounds, most notably tetrahydrocannabinol (THC) and cannabidiol (CBD). These cannabinoids interact with the body's endocannabinoid system and can affect various physiological processes, including cardiovascular function. While there are no large-scale clinical trials specifically examining the combined use of cannabis and sildenafil, both substances independently affect the cardiovascular system, which raises theoretical concerns about their concurrent use.

It's important to note that different cannabis products may have varying interaction potential. THC-dominant recreational cannabis, over-the-counter CBD products, and prescribed cannabis-based medicinal products (CBPMs) have different compositions and potential effects. The absence of evidence is not evidence of safety. Healthcare professionals should be aware that patients may not routinely disclose cannabis use, and proactive questioning about all substances—including recreational drugs—is essential when prescribing medications like sildenafil.

Potential Risks of Combining Cannabis with Sildenafil

Whilst definitive clinical evidence is limited, several theoretical and reported risks warrant consideration when cannabis and sildenafil are used together. The primary concerns relate to cardiovascular effects, altered drug metabolism, and unpredictable physiological responses.

Cannabis use, particularly THC-containing products, commonly increases heart rate by 20–50 beats per minute within minutes of consumption. While sildenafil typically causes minimal changes in heart rate, tachycardia and palpitations can occur as uncommon side effects. When combined, these effects could potentially place additional strain on the cardiovascular system, particularly in individuals with pre-existing cardiac conditions.

Dizziness and orthostatic hypotension (a sudden drop in blood pressure upon standing) represent another significant concern. Both substances can cause these effects independently, and their combination may increase the risk of falls, syncope (fainting), or other injuries. This risk is particularly elevated in older patients, those taking alpha-blockers or other antihypertensive medications, or people with autonomic dysfunction.

It is crucial to note that sildenafil must never be used with nitrates (including recreational 'poppers') or riociguat due to the risk of severe, potentially life-threatening hypotension. This contraindication is absolute and highlighted in all UK prescribing guidance.

Regarding drug metabolism, some components of cannabis, particularly high-dose oral CBD, can inhibit cytochrome P450 enzymes, including CYP3A4. Since sildenafil is metabolised by this enzyme system, concurrent use of CBD products could theoretically alter sildenafil levels in the bloodstream. The clinical significance of this interaction remains unclear and likely depends on the dose, frequency, and formulation of cannabis used.

There are case reports of cardiovascular events with either cannabis or sildenafil individually; however, evidence specifically for combined use is sparse, and causality cannot be definitively established due to multiple confounding factors.

Blood Pressure Effects: Cannabis and Sildenafil Interaction

The cardiovascular effects of combining cannabis and sildenafil deserve particular attention, as both substances exert complex and sometimes contradictory effects on blood pressure regulation. Understanding these mechanisms is essential for assessing individual patient risk.

Sildenafil causes vasodilation through its inhibition of PDE5, leading to increased levels of cyclic guanosine monophosphate (cGMP) in smooth muscle cells. This results in relaxation of vascular smooth muscle and typically causes a mild reduction in systolic and diastolic blood pressure—usually around 8–10 mmHg systolic and 5–6 mmHg diastolic in normotensive individuals. These effects are generally well-tolerated but can be clinically significant in certain situations.

It's important to note that while sildenafil is not contraindicated with alpha-blockers, caution is advised. Patients should be stable on alpha-blocker therapy before starting sildenafil, which should be initiated at the lowest dose (25 mg), with consideration given to separating the doses to minimise hypotension risk. In contrast, sildenafil is absolutely contraindicated with nitrates and riociguat due to severe hypotension risk.

Cannabis exhibits biphasic cardiovascular effects that vary with dose, route of administration, and individual tolerance. Acute cannabis use typically causes an initial increase in heart rate and blood pressure, followed by a potential decrease in blood pressure, particularly in the standing position. Chronic cannabis users may develop tolerance to some of these effects, but the cardiovascular response remains unpredictable.

The combination of these two substances creates a scenario where blood pressure may fluctuate unpredictably. The initial sympathomimetic effects of cannabis (increased heart rate and blood pressure) may be followed by enhanced vasodilation when combined with sildenafil's effects, potentially leading to significant hypotension. This is particularly concerning during position changes, physical exertion, or sexual activity—the very context in which sildenafil is typically used.

Patients with underlying cardiovascular disease, including coronary artery disease, heart failure, or poorly controlled hypertension, face elevated risks. The MHRA advises caution with sildenafil in patients with cardiovascular risk factors (as noted in the product SmPC), and the addition of cannabis—a substance with unpredictable cardiovascular effects—compounds these concerns.

Safe Use: What to Tell Your Doctor

Open and honest communication with healthcare professionals is paramount when considering or currently using sildenafil, particularly if you also use cannabis. Many patients feel uncomfortable discussing recreational drug use, but this information is essential for safe prescribing and monitoring.

When consulting your GP or prescriber about sildenafil, you should disclose:

• All cannabis use, including frequency, amount, method of consumption (smoking, vaping, edibles), and THC/CBD content if known

• Any cardiovascular symptoms you experience with either substance, such as chest pain, palpitations, dizziness, or shortness of breath

• All other medications, including over-the-counter products and supplements

• Any history of cardiovascular disease, including hypertension, angina, previous heart attack, or stroke

• Episodes of fainting, severe dizziness, or falls

Your doctor cannot provide safe, effective care without complete information, and discussions about substance use are confidential and non-judgemental. Healthcare professionals are concerned with your safety, not making moral judgements about lifestyle choices.

If you are currently using both substances, be alert for warning signs that require immediate medical attention:

• Chest pain or pressure – call 999 immediately, particularly during or after sexual activity

• Severe dizziness or fainting – seek urgent medical attention

• Irregular or very rapid heartbeat – contact NHS 111 or your GP urgently

• Severe headache – seek medical advice

• Vision changes or sudden vision loss – go to A&E immediately

• Prolonged erection lasting more than four hours (priapism) – go to A&E immediately

Never use sildenafil with nitrates (including recreational 'poppers') or riociguat as this combination can cause dangerous drops in blood pressure.

NICE guidance emphasises the importance of cardiovascular risk assessment before initiating PDE5 inhibitors. If you use cannabis regularly, this should form part of your risk assessment. Your doctor may recommend cardiovascular investigations, adjust dosing, or suggest alternative treatments based on your individual circumstances.

Consider discussing harm reduction strategies, such as avoiding concurrent use, using lower doses of sildenafil (starting at 25 mg), or spacing the timing between cannabis and sildenafil use. Remember that sildenafil is available through legitimate NHS and private prescribing routes—avoid obtaining it from unregulated sources, as this increases risks further.

If you experience any suspected side effects from sildenafil, cannabis, or their combination, report them through the MHRA Yellow Card scheme, which helps monitor medication safety.

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