Antispasmodic drugs are a huge moneyspinner for drugs companies. Sometimes, they are teamed with a tranquilliser like Librium, under the premise that a drug that relaxes all of you will also calm down your gut. According to the Health Research Group (HRG) in the US, this category of drug accounts for some four million prescriptions for older Americans alone. HRG calls this an 'irrational mixture of drugs' and urges patients not to use them.
The irony is that not one of these patients should be given an antispasmodic of any variety. Indeed, your mother's doctor had no business prescribing this drug to her in the first place. In the Medicines Compendium, Norgine clearly states that Spasmonal should not be given to anyone over 40. They are also not supposed to be used except for the short term and only when pain is present.
Also, anyone who has lost his appetite or lost weight recently, feels tired or has severe constipation shouldn't use these drugs.
Instead of a drug, your mother could try peppermint oil, which naturally relaxes the gut, or try to work out the cause of her IBS. An irritable bowel is a sign of a digestive disorder. First, suspect allergies, low stomach acid or pancreatic enzymes, or parasites. See The WDDTY Guide to Good Digestion for treatments.