Sildenafil Citrate is the citrate salt of sildenafil and is a well-established pharmaceutical active ingredient belonging to the phosphodiesterase type-5 (PDE5) inhibitor class. It is used in pharmaceutical formulations for erectile dysfunction and pulmonary arterial hypertension. FDA documentation identifies sildenafil citrate as the 1:1 citrate salt of sildenafil and gives a molecular weight of approximately 666.7 g/mol.
The API is supplied as a white or almost white, slightly hygroscopic crystalline powder. It is slightly soluble in water and methanol and practically insoluble in hexane.
Sildenafil Citrate has the molecular formula C₂₂H₃₀N₆O₄S·C₆H₈O₇, corresponding to C₂₈H₃₈N₆O₁₁S, with a molecular weight of 666.70 g/mol. The sildenafil active moiety has a molecular weight of approximately 474.58 g/mol.
Key identity information:
Sildenafil citrate is a synthetic small-molecule pharmaceutical salt formed from sildenafil and citric acid in a 1:1 stoichiometric relationship. The citrate salt is the pharmaceutical form used in several approved sildenafil products.
The API is chemically characterized by spectroscopic and chromatographic methods. USP identification includes infrared spectroscopy, while assay is performed by liquid chromatography using UV detection.
Sildenafil selectively inhibits cyclic guanosine monophosphate-specific phosphodiesterase type 5 (PDE5). Inhibition of PDE5 reduces the degradation of cGMP, resulting in increased intracellular cGMP and relaxation of smooth muscle.
In pulmonary vasculature, this mechanism produces pulmonary vasodilation. In erectile tissue, increased cGMP signaling contributes to smooth-muscle relaxation and facilitates erection in response to sexual stimulation.
Sildenafil citrate is used as an API in pharmaceutical formulations for:
FDA-approved sildenafil products include formulations containing sildenafil citrate equivalent to defined amounts of sildenafil.
Sildenafil Citrate has an established USP monograph with defined chemical quality requirements.
USP 2025 specifies:
The assay is calculated on an anhydrous and solvent-free basis.
Sildenafil Citrate is manufactured through multi-step chemical synthesis followed by purification and controlled salt formation/crystallization. Regulatory assessment documents describe the drug substance as a chemically synthesized API with validated analytical controls.
Manufacturing controls include reaction purity, residual solvents, impurity removal, crystallization, drying and final pharmaceutical-grade testing.
Sildenafil Citrate is a white or almost white, slightly hygroscopic crystalline powder. It is slightly soluble in water and methanol and practically insoluble in hexane.
FDA's original chemistry review reports a melting range of approximately 191–202°C for sildenafil citrate.
Sildenafil citrate does not contain chiral centers, so specific optical rotation is not an appropriate universal release parameter. Regulatory assessment also confirms that polymorphism has been observed for the drug substance.
USP specifies that Sildenafil Citrate should be preserved in airtight containers and stored at room temperature.
Moisture protection is important because the material is slightly hygroscopic and the USP assay is calculated on an anhydrous and solvent-free basis.
A pharmaceutical-grade Sildenafil Citrate API documentation package may include:
Sildenafil Citrate has established pharmacopoeial quality requirements, making control of assay, water, residue on ignition, imidazole, related substances and total impurities essential for pharmaceutical-grade material. USP 2025 provides a clearly defined specification framework for the API.
| Parameter | Specification |
|---|---|
| Appearance | white to off-white crystalline powder |
| Identification | IR & HPLC compliant |
| Assay (HPLC) | 98.0% – 102.0% |
| Residue on Ignition | 191–202°C |
| Total Impurities | NMT 0.5% |
| Water Content | NMT 2.5% |
| Melting Point | 191–202°C |