Just taken this off Medscape.com (part of WebMD) which explains that it is supposed to do this:
Venlafaxine Hydrochloride (Efexor XL)
Originally, venlafaxine was brought to the market as a sustained-release formulation, Efexor XL by Wyeth, now a subsidiary company of Pfizer. Efexor XL uses a diffusion mechanism. The active drug is on film-coated spheroids, which are packed into capsules. On ingestion, the outer cover of the capsule completely dissolves in the gastric fluids releasing the spheroids. The spheroids are made of inert insoluble ball-like figures on which the active drug is applied. The spheroids are additionally covered with a porous insoluble polymer (ethylcellulose) that acts as a semipermeable membrane through which the drug diffuses once in contact with GI fluids (Data on file 59, Pfizer). Spheroids may be visible in the stool after the medication has been used.
Prescribing information on Efexor XL states: 'Venlafaxine prolonged-release capsules contain spheroids, which release the active substance slowly into the digestive tract. The insoluble portion of these spheroids is eliminated and may be seen in faeces' [Pfizer, 2011a].
The patient information leaflet states the following under the side effect section:
Do not be concerned if you see small white balls or granules in your stool after taking this medicine … as they travel through your stomach and intestines, venlafaxine is slowly released. The spheroid 'shell' does not dissolve and is passed out in your stools. So even though you may see spheroids in your stools, your dose of medicine has been absorbed [Pfizer, 2011b].
Several brands of venlafaxine extended-release formulations made with different technologies are available today. Wyeth also produced extended-release tablets of desvenlafaxine, an active metabolite of venlafaxine under the brand name Pristiq available in the USA and Canada. The patient information leaflet states the following under the heading 'How should I take Pristiq®?': 'When you take Pristiq®, you may see something in your stool that looks like a tablet. This is the empty shell from the tablet after the medicine has been absorbed by your body' [Pfizer, 2011c]. Mrs Y was taking Venlalic, a generic product that uses an osmotic release mechanism; hence it is expected to have an intact empty shell left behind.