“Over the counter herbal sex enhancers may contain PDE5 inhibitors or prescription-only Sildenafil drugs, clinically indicated for the treatment of erectile dysfunction,” James Wee, Pfizer Philippines senior medical manager, said in a statement.

“PDE5 inhibitors are never bought as over-the-counter medicines. It should only be purchased with a prescription from a physician,” he said.

The warning was issued after global sales of Viagra fell by 9% to $499 million in 2011 mostly due to competition from other brands.

Pfizer holds the patent for Viagra in the United States until 2019.

“Counterfeits and clones shorten or circumvent the proper process of research and development often compromising efficacy, quality, and safety of drugs. There are other processes that can be used to produce Sildenafil but safety and efficacy of Sildenafil produced by alternative, substandard processes are unproven,” said Mr. Wee.

Pfizer noted the public should familiarize themselves with packaging of original drugs and must always purchase medicine from licensed drugstores.

The firm said prints on labels must be clear and must not have been tampered and must show the generic name of the product.

Herbal medicines are allowed to be sold by the Health department but need to be clearly marked that these have not been scientifically proven to be effective.

According to earlier reports, Pfizer expects its global revenues to come at $63 billion to $65.5 billion, down from its previously bared outlook of $65.2 billion to $67.7 billion.

The firm recently cut its 2012 research and development budget to $6.5 billion to $7 billion from $8 billion to $8.5 billion to save on costs.

It also plans to shut down its research center in Sandwich, England and will lay off around 1,000 employees.

Pfizer shares are traded on the New York Stock Exchange and is said to be the world’s largest pharmaceutical company in terms of revenues.

It reportedly started selling Viagra in 1998. -- Emilia Narni J. David