Paroxetine HCl is an orally administered antidepressant with a chemical structure unrelated to other selective serotonin reuptake inhibitors or to tricyclic, tetracyclic or other available antidepressant agents. It is the hydrochloride salt of a phenylpiperidine compound identified chemically as (Immediate-Release Tablets and Oral Suspension:) (-)-trans-4R-(4'-fluorophenyl)-3S-[(3',4'-methylenedioxyphenoxy) methyl] piperidine hydrochloride hemihydrate and (Controlled-Release Tablets:) (-) - (3S,4R)-4-[(p-fluorophenyl)-3-[(3,4-methylenedioxy) phenoxy]methyl]piperidine hydrochloride hemihydrate.

The empirical formula is C₁₉H₂₀FNO₃•HCl•½H₂O. The molecular weight is 374.8 (329.4 as free base).

Paroxetine HCl is an odorless, off-white powder, having a melting point range of 120-138°C and a solubility of 5.4 mg/ml in water.

Immediate-Release Tablets: Each film-coated Paxil tablet contains paroxetine HCl equivalent to paroxetine as follows: 10 mg: Yellow; 20 mg: Pink (scored); 30 mg: Blue, 40 mg: Green. Inactive ingredients consist of dibasic calcium phosphate dihydrate, hydroxypropyl methylcellulose, magnesium stearate, polyethylene glycols, polysorbate 80, sodium starch glycolate, titanium dioxide and one or more of the following: D&C red no. 30, D&C yellow no. 10, FD&C blue no. 2, FD&C yellow no. 6.

Oral Suspension: Each 5 ml of Paxil orange-colored, orange-flavored liquid contains paroxetine HCl equivalent to paroxetine, 10 mg. Inactive ingredients consist of polacrilin potassium, microcrystalline cellulose, propylene glycol, glycerin, sorbitol, methyl paraben, propyl paraben, sodium citrate dihydrate, citric acid anhydrate, sodium saccharin, flavorings, FD&C yellow no. 6 and simethicone emulsion.

Controlled-Release Tablets: Each enteric, film-coated, bilayer, controlled-release Paxil tablet contains paroxetine HCl equivalent to paroxetine as follows: 12.5 mg and 25 mg. One layer of the tablet consists of a degradable barrier layer and the other contains the active material in a hydrophilic matrix. The barrier layer is pale yellow and pink for the 12.5 mg and 25 mg strength tablets, respectively; the active layer is white.

Inactive ingredients consist of hydroxypropyl methylcellulose, polyvinylpyrrolidone, lactose monohydrate, magnesium stearate, colloidal silicon dioxide, glyceryl behenate, methacrylic acid copolymer type C, sodium lauryl sulfate, polysorbate 80, talc, triethyl citrate, and one or more of the following colorants: yellow ferric oxide, red ferric oxide.