The IV is the new "it" bag. Don't leave home without it. CHRISTINA Andrews, 48, is lounging in an oversize beige leather chair in the Park Avenue South office of Dr. Jeffrey Morrison, who specializes in nutrition and integrative medicine. It's a Wednesday afternoon, and she's checking her e-mail and relaxing. With her perfectly coiffed blond hair and an Hermès belt cinching her waist, the chic Murray Hill resident looks like she's ready for lunch at Il Mulino - except for the thin tube connected to her arm. "I get B-12 and other vitamins, and I come for two hours, sometimes three, because I like to take them slow,"" she explains of her monthly IVs that run about $130 per session. Local NYC doctors are seeing a spike in requests for intravenous vitamin treatments. "I'm a little tired immediately after, but the next day, I feel I can do anything."" Andrews is one of a growing number of New Yorkers - from the night-life crowd to the yoga community - who have been getting intravenous cocktails to keep up their energy, combat colds, stay youthful or simply look better. Though they are not FDA-approved as treatments, IVs are being administered at the offices of even prominent physicians. In many cases, patients first begin with a blood workup, to determine what nutrients they need. Then, they sit with a drip from 30 minutes to an hour, at a cost ranging from $130 to $1,000 per session. "It's basic biochemistry; when the body has its building blocks, it works better," says Morrison, who recommends weekly drips during particularly stressful periods for a span of four to six weeks. According to Morrison, IVs first started to become popular with athletes about five years ago, when Major League Baseball players were rumored to use them, because the treatments allowed them to enhance their performance legally. The financial community then followed suit. "People on Wall Street are working long hours, and they are looking for a natural way to sustain a high level of functioning," says Morrison, who frequently administers a "Myers" Cocktail"" of calcium, the trace mineral selenium, vitamin C and B-complex (it's named after the physician, John Myers, who invented it). Though doctors in a large range of specialties are now offering the IVs, critics say they are nothing more than snake-oil salesmen. "There is no evidence-based medicine to support the use of vitamin drips; they are just moneymakers," says Elizabeth Kavaler, a urologist and assistant clinical professor at Weill Cornell Medical Centre.