Psychiatric drug-related weight gain "is a huge problem," Dr. David Lau, chairman of the diabetes and endocrine research group at the University of Calgary and president of Obesity Canada, told The Ottawa Citizen.

"You can see patients gaining 10, 20, 30, 40 pounds."

Not everyone taking the psychiatric medications will gain weight, but patients may not be warned of the possible weight gain.

Lau said the anti-psychotics most likely to be associated with weight gain are: Clozaril, or clozapine; Zyprexa, Seroquel or quetiapine; Risperdal, or risperidone; Modecate or chlorpromazine; fluphenazine and Haldol or haloperidol.

How the drugs may cause weight gain isn't well understood, but some may stimulate appetite, while others may unmask a person's genetic propensity to gain weight, or cause the body to become resistant to insulin, Lau explained.