Mechanism of Action: How Sildenafil Works

In patients who exhibit hemodynamic instability while receiving an α-adrenergic blocking agent, use caution. [1] Patients should be stable on an α-adrenergic blocking agent prior to initiation of sildenafil, and sildenafil should be administered at the lowest possible dose. [1] In patients receiving an optimal dose of sildenafil, initiate the α-adrenergic blocking agent at the lowest dose.

Concomitant administration of ritonavir substantially increases serum concentrations of sildenafil (11-fold increase in AUC).

• The drug’s effectiveness can vary based on individual health.
• Use caution when engaging in sexual activity with underlying health issues.
• Sildenafil can cause visual disturbances like blue tinting.
• Do not use sildenafil if you have a recent history of stroke or MI.

[1] Data are limited; decreased blood pressure, syncope, and prolonged erection have been reported in some healthy volunteers exposed to high doses of sildenafil (200-800 mg). [1] Use sildenafil with caution in patients receiving ritonavir; reduced sildenafil dosage is recommended to decrease the chance of adverse reactions to sildenafil.

Bleeding events have been reported in patients taking sildenafil for ED. [1] In patients with bleeding disorders or active peptic ulcers, sildenafil should be used with caution since safety of the drug has not been established.

Related Product

[1] [26][67] At clinically relevant concentrations, the manufacturer states that it is unlikely that sildenafil will alter the clearance of drugs metabolized by these isoenzymes. A minor route of metabolism of sildenafil is through the CYP2C9 isoenzyme. [1][67][250] Although some clinicians state that the possibility of an interaction between sildenafil and drugs metabolized by CYP2C9 should be considered, there was no evidence of appreciable inhibition of CYP2C9-mediated (e.g., tolbutamide, warfarin) or CYP3A4-mediated (e.g., ritonavir, saquinavir) metabolism by sildenafil in clinical studies. **Antacids:**Single doses of an aluminum and magnesium hydroxide-containing antacid did not affect the oral bioavailability of sildenafil. Cimetidine: Plasma sildenafil concentrations increased by approximately 56% in healthy individuals who received a single 50-mg oral dose of the drug concomitantly with a single oral dose of cimetidine (800 mg), a nonspecific inhibitor of the cytochrome P-450 mixed-function oxidase system. [1][27][33][67][131] The possibility that sildenafil could potentiate the effects of certain other drugs exhibiting antiplatelet activity should be considered. Safety and efficacy have not been established for use of sildenafil in combination with other PDE type 5 inhibitors or other treatments for ED; such combinations may further lower blood pressure and are not recommended.

Patients should be advised that use of sildenafil provides no protection against sexually transmitted diseases and they should be counseled regarding protective measures to guard against such transmission. Sildenafil for ED (e.g., Viagra®) is not indicated for use in females. [1] There are no data with use in pregnant women to inform any drug-associated risks for adverse developmental outcomes. No evidence of teratogenicity, embryotoxicity, or fetotoxicity was observed in rats and rabbits receiving up to 200 mg/kg daily of sildenafil during organogenesis.

Overdose/Missed Dose

[1][67] Population pharmacokinetic analysis of data from clinical trials indicates that cimetidine reduces sildenafil clearance when these drugs are administered concomitantly. [1][139] Some clinicians recommend that a lower initial sildenafil dose (25 mg) be considered in patients with ED receiving cimetidine. Concomitant use of any form of organic nitrate (e.g., nitroglycerin), including recreational use of inhaled nitrites (amyl nitrate or nitrite, ''poppers''), with sildenafil is contraindicated due to the potential pharmacodynamic interaction (increased hypotensive effect). However, the American College of Cardiology (ACC) and American Heart Association (AHA) recognize that use of organic nitrates and nitrites in patients receiving sildenafil may not be completely avoidable, provided sufficient time has elapsed between use of sildenafil and administration of the nitrate or nitrite. [67] Although it is not known how much time must elapse between use of sildenafil and administration of a nitrate or nitrite, pharmacokinetic data suggest that these latter agents should not be given within 24 hours of sildenafil administration because an exaggerated hypotensive response is likely; plasma sildenafil concentrations are substantially lower 24 hours after a dose than peak concentrations. [1] These doses in rats and rabbits represent about 16 and 32 times, respectively, the maximum recommended human dose (MRHD) for the treatment of ED on a mg/m2 basis in a 50-kg patient.

Drug Interactions

In patients who exhibit hemodynamic instability while receiving an α-adrenergic blocking agent, use caution. [1] Patients should be stable on an α-adrenergic blocking agent prior to initiation of sildenafil, and sildenafil should be administered at the lowest possible dose. [1] In patients receiving an optimal dose of sildenafil, initiate the α-adrenergic blocking agent at the lowest dose. Concomitant administration of ritonavir substantially increases serum concentrations of sildenafil (11-fold increase in AUC). [1] Data are limited; decreased blood pressure, syncope, and prolonged erection have been reported in some healthy volunteers exposed to high doses of sildenafil (200-800 mg).

Dosage for pulmonary arterial hypertension (PAH)

[1] Use sildenafil with caution in patients receiving ritonavir; reduced sildenafil dosage is recommended to decrease the chance of adverse reactions to sildenafil. Bleeding events have been reported in patients taking sildenafil for ED. [1] In patients with bleeding disorders or active peptic ulcers, sildenafil should be used with caution since safety of the drug has not been established. [1][27][33][67][131] The possibility that sildenafil could potentiate the effects of certain other drugs exhibiting antiplatelet activity should be considered. Safety and efficacy have not been established for use of sildenafil in combination with other PDE type 5 inhibitors or other treatments for ED; such combinations may further lower blood pressure and are not recommended. [1] No adverse effects were observed Iin a prenatal and postnatal development study in rats receiving 30 mg/kg daily for 36 days (equivalent to 2-times the MRHD on a mg/m2 basis in a 50-kg subject). [1] Limited data indicate that sildenafil and its active metabolite are present in human milk. [1] There is no information about the effects of sildenafil on the breastfed infant or on milk production. Reproduction studies revealed no evidence of impaired fertility at sildenafil dosages up to 60 mg/kg daily (for 36 days in female rats and 102 days in male rats), a dosage representing more than 25 times the human male AUC. [1] No effect on sperm motility or morphology was noted after a single 100-mg oral sildenafil dose in healthy human adults. Sildenafil for ED (e.g., Viagra®) is not indicated for use in pediatric patients. [1] The manufacturer states that safety and efficacy of sildenafil in children have not been established. The AUC of free (unbound) sildenafil and its active N-desmethyl metabolite were 45 and 57% higher, respectively, in healthy volunteers >=65 years of age compared to healthy volunteers 18-45 years sildenafil tables of age. [1] Clinical studies included patients >=65 years of age (18%) and >=75 years of age (2%); no overall differences in safety and efficacy were observed between older (>=65 years of age) and younger (<65 years of age) patients[1] Because higher plasma levels may increase the incidence of adverse reactions, consider reducing the initial sildenafil dosage to 25 mg in older patients. In patients with hepatic cirrhosis (Child-Pugh class A or B), sildenafil clearance is reduced, resulting in increased AUC (by 85%) and peak plasma concentrations (by 47%) compared with values observed in age-matched healthy adults. [1][31][131] The effect of severe hepatic impairment on the pharmacokinetics of sildenafil has not been evaluated to date. In patients with any degree of hepatic impairment (e.g. cirrhosis), consider reducing the initial dose of sildenafil to 25 mg.[1] In patients with mild (creatinine clearance 50-80 mL/minute) or moderate (creatinine clearance 30-49 mL/minute) renal impairment, the pharmacokinetics of a single 50-mg oral dose of sildenafil are not altered. [1][31][131] However, in patients with severe (creatinine clearance less than 30 mL/minute) renal impairment, sildenafil clearance is reduced, resulting in AUC and peak plasma concentrations of the parent drug that are approximately double those in age-matched healthy adults. [1][31][67][131] In addition, AUC and peak plasma concentrations of the N-demethylated metabolite are 200 and 79% greater, respectively, than those in individuals with normal renal function. In patients with severe renal impairment (creatinine clearance less than 30 mL/minute), consider reducing the initial dose of sildenafil to 25 mg.[1] The most common adverse effects (>=2%) of sildenafil used for the treatment of ED include headache, flushing, dyspepsia, abnormal vision, nasal congestion, back pain, myalgia, nausea, dizziness, and rash. Sildenafil is metabolized principally via hepatic cytochrome P-450 (CYP) microsomal isoenzymes 3A4 (major route) and 2C9 (minor route). [1][91][131] Inhibitors and inducers of these isoenzymes may reduce or increase sildenafil clearance, respectively. In vitro studies indicate that sildenafil is sildenafil online ireland a weak inhibitor of the CYP isoenzymes 1A2, 2C9, 2C19, 2D6, 2E1, and 3A4.

• Be aware of the signs of adverse reactions like chest pain.
• Do not use sildenafil if you are taking medications for heart issues.
• The medication does not cause sexual arousal; stimulation is needed.
• Use only the prescribed dose, especially at high levels like 150 mg.

[1][26][67][250] Sildenafil is not expected to affect the pharmacokinetics of substrates of these CYP enzymes at clinically relevant concentrations.

• Sildenafil citrate tablets are used to treat erectile dysfunction.
• The typical dosage of sildenafil citrate is 150 mg.
• These tablets work by increasing blood flow to the penis.
• Consult a doctor before taking 150 mg sildenafil tablets.

The possibility that any drug that inhibits CYP3A4 may interact with sildenafil should be considered.

Call to Action for Buyers and Distributors

[1][29][31][67] The point at which nitrates or nitrites can be given safely is unclear, and therefore the drugs should be avoided unless, in the view of the treating clinician, the benefits outweigh the risks. If consideration is given to administering a nitrate or nitrite beyond 24 hours after sildenafil use, the response to the initial doses must be monitored carefully and proper facilities for fluid and vasopressor (e.g., α-adrenergic agonists) support must be readily available to prevent acute ischemic episodes. [67][159] In patients in whom clearance of sildenafil and/or its metabolites may be prolonged (e.g., those with hepatic [e.g., cirrhosis] or severe renal impairment [e.g., creatinine clearance less than 30 mL/minute], those receiving a potent inhibitor of CYP3A4, geriatric patients older than 65 years of age), a more extended period of time between use of sildenafil and administration of a nitrate or nitrite may be necessary. [1][67][154] In either case, a short-acting nitrate formulation that can be titrated readily (e.g., IV nitroglycerin) is preferred and such use should be accompanied by close hemodynamic monitoring. [1][67] In some cases, a reduction in sildenafil dosage is recommended. Clinically important pharmacokinetic interactions have been reported with several antiretroviral agents that inhibit CYP3A4 (e.g., ritonavir, saquinavir) and can potentially result in an increase in sildenafil-associated adverse effects. The possibility that any drug that induces CYP3A4 may interact with sildenafil should be considered. Pharmacokinetic data from patients in clinical trials showed no effect on sildenafil pharmacokinetics when the drug was used concomitantly with CYP2D6 inhibitors such as selective serotonin-reuptake inhibitors (SSRIs) or tricyclic antidepressants.

What special dietary instructions should I follow?

Sildenafil for ED (e.g., Viagra®) is not indicated for use in pediatric patients. [1] The manufacturer states that safety and efficacy of sildenafil in children have not been established. The AUC of free (unbound) sildenafil and its active N-desmethyl metabolite were 45 and 57% higher, respectively, in healthy volunteers >=65 years of age compared to healthy volunteers 18-45 years sildenafil tables of age. [1] Clinical studies included patients >=65 years of age (18%) and >=75 years of age (2%); no overall differences in safety and efficacy were observed between older (>=65 years of age) and younger (<65 years of age) patients[1] Because higher plasma levels may increase the incidence of adverse reactions, consider reducing the initial sildenafil dosage to 25 mg in older patients. In patients with hepatic cirrhosis (Child-Pugh class A or B), sildenafil clearance is reduced, resulting in increased AUC (by 85%) and peak plasma concentrations (by 47%) compared with values observed in age-matched healthy adults.

Vitamin K Antagonists

[1][31][131] The effect of severe hepatic impairment on the pharmacokinetics of sildenafil has not been evaluated to date. In patients with any degree of hepatic impairment (e.g. cirrhosis), consider reducing the initial dose of sildenafil to 25 mg.[1] In patients with mild (creatinine clearance 50-80 mL/minute) or moderate (creatinine clearance 30-49 mL/minute) renal impairment, the pharmacokinetics of a single 50-mg oral dose of sildenafil are not altered. [1][31][131] However, in patients with severe (creatinine clearance less than 30 mL/minute) renal impairment, sildenafil clearance is reduced, resulting in AUC and peak plasma concentrations of the parent drug that are approximately double those in age-matched healthy adults. [1][31][67][131] In addition, AUC and peak plasma concentrations of the N-demethylated metabolite are 200 and 79% greater, respectively, than those in individuals with normal renal function. [1] [26][67] At clinically relevant concentrations, the manufacturer states that it is unlikely that sildenafil will alter the clearance of drugs metabolized by these isoenzymes. A minor route of metabolism of sildenafil is through the CYP2C9 isoenzyme. [1][67][250] Although some clinicians state that the possibility of an interaction between sildenafil and drugs metabolized by CYP2C9 should be considered, there was no evidence of appreciable inhibition of CYP2C9-mediated (e.g., tolbutamide, warfarin) or CYP3A4-mediated (e.g., ritonavir, saquinavir) metabolism by sildenafil in clinical studies. **Antacids:**Single doses of an aluminum and magnesium hydroxide-containing antacid did not affect the oral bioavailability of sildenafil. Cimetidine: Plasma sildenafil concentrations increased by approximately 56% in healthy individuals who received a single 50-mg oral dose of the drug concomitantly with a single oral dose of cimetidine (800 mg), a nonspecific inhibitor of the cytochrome P-450 mixed-function oxidase system.

Can You Take Sildenafil on An Empty Stomach?

Patients should be advised that use of sildenafil provides no protection against sexually transmitted diseases and they should be counseled regarding protective measures to guard against such transmission. Sildenafil for ED (e.g., Viagra®) is not indicated for use in females. [1] There are no data with use in pregnant women to inform any drug-associated risks for adverse developmental outcomes. No evidence of teratogenicity, embryotoxicity, or fetotoxicity was observed in rats and rabbits receiving up to 200 mg/kg daily of sildenafil during organogenesis. [1] These doses in rats and rabbits represent about 16 and 32 times, respectively, the maximum recommended human dose (MRHD) for the treatment of ED on a mg/m2 basis in a 50-kg patient.

Jet lag research

[1] No adverse effects were observed Iin a prenatal and postnatal development study in rats receiving 30 mg/kg daily for 36 days (equivalent to 2-times the MRHD on a mg/m2 basis in a 50-kg subject). [1] Limited data indicate that sildenafil and its active metabolite are present in human milk. [1] There is no information about the effects of sildenafil on the breastfed infant or on milk production. Reproduction studies revealed no evidence of impaired fertility at sildenafil dosages up to 60 mg/kg daily (for 36 days in female rats and 102 days in male rats), a dosage representing more than 25 times the human male AUC. [1] No effect on sperm motility or morphology was noted after a single 100-mg oral sildenafil dose in healthy human adults. [1][67] Population pharmacokinetic analysis of data from clinical trials indicates that cimetidine reduces sildenafil clearance when these drugs are administered concomitantly. [1][139] Some clinicians recommend that a lower initial sildenafil dose (25 mg) be considered in patients with ED receiving cimetidine. Concomitant use of any form of organic nitrate (e.g., nitroglycerin), including recreational use of inhaled nitrites (amyl nitrate or nitrite, ''poppers''), with sildenafil is contraindicated due to the potential pharmacodynamic interaction (increased hypotensive effect). However, the American College of Cardiology (ACC) and American Heart Association (AHA) recognize that use of organic nitrates and nitrites in patients receiving sildenafil may not be completely avoidable, provided sufficient time has elapsed between use of sildenafil and administration of the nitrate or nitrite. [67] Although it is not known how much time must elapse between use of sildenafil and administration of a nitrate or nitrite, pharmacokinetic data suggest that these latter agents should not be given within 24 hours of sildenafil administration because an exaggerated hypotensive response is likely; plasma sildenafil concentrations are substantially lower 24 hours after a dose than peak concentrations. [1][29][31][67] The point at which nitrates or nitrites can be given safely is unclear, and therefore the drugs should be avoided unless, in the view of the treating clinician, the benefits outweigh the risks. If consideration is given to administering a nitrate or nitrite beyond 24 hours after sildenafil use, the response to the initial doses must be monitored carefully and proper facilities for fluid and vasopressor (e.g., α-adrenergic agonists) support must be readily available to prevent acute ischemic episodes. [67][159] In patients in whom clearance of sildenafil and/or its metabolites may be prolonged (e.g., those with hepatic [e.g., cirrhosis] or severe renal impairment [e.g., creatinine clearance less than 30 mL/minute], those receiving a potent inhibitor of CYP3A4, geriatric patients older than 65 years of age), a more extended period of time between use of sildenafil and administration of a nitrate or nitrite may be necessary.

Before Using

In patients with severe renal impairment (creatinine clearance less than 30 mL/minute), consider reducing the initial dose of sildenafil to 25 mg.[1] The most common adverse effects (>=2%) of sildenafil used for the treatment of ED include headache, flushing, dyspepsia, abnormal vision, nasal congestion, back pain, myalgia, nausea, dizziness, and rash. Sildenafil is metabolized principally via hepatic cytochrome P-450 (CYP) microsomal isoenzymes 3A4 (major route) and 2C9 (minor route). [1][91][131] Inhibitors and inducers of these isoenzymes may reduce or increase sildenafil clearance, respectively. In vitro studies indicate that sildenafil is sildenafil online ireland a weak inhibitor of the CYP isoenzymes 1A2, 2C9, 2C19, 2D6, 2E1, and 3A4. [1][26][67][250] Sildenafil is not expected to affect the pharmacokinetics of substrates of these CYP enzymes at clinically relevant concentrations.

Recreational use

The possibility that any drug that inhibits CYP3A4 may interact with sildenafil should be considered. [1][67] In some cases, a reduction in sildenafil dosage is recommended. Clinically important pharmacokinetic interactions have been reported with several antiretroviral agents that inhibit CYP3A4 (e.g., ritonavir, saquinavir) and can potentially result in an increase in sildenafil-associated adverse effects. The possibility that any drug that induces CYP3A4 may interact with sildenafil should be considered. Pharmacokinetic data from patients in clinical trials showed no effect on sildenafil pharmacokinetics when the drug was used concomitantly with CYP2D6 inhibitors such as selective serotonin-reuptake inhibitors (SSRIs) or tricyclic antidepressants. [1][67][154] In either case, a short-acting nitrate formulation that can be titrated readily (e.g., IV nitroglycerin) is preferred and such use should be accompanied by close hemodynamic monitoring.