Why am I suddenly lactose intolerant?

If dairy never bothered you and now it does, there's a real explanation. Lactose intolerance really can come on suddenly in adulthood, and the usual reason is a temporary one that settles again once your gut recovers.

Two different kinds of lactose intolerance

It helps to know there are two routes to it:

• Primary — the common, genetic kind. Your body naturally makes less of the lactase enzyme as you move out of childhood, so symptoms can quietly begin in your teens or adult years. This is gradual and permanent.
• Secondary (acquired) — caused by something that damages the lining of your small intestine, where lactase is made. This is the one that can appear suddenly, and it's often temporary.

What triggers a sudden onset

Secondary lactose intolerance follows injury to the gut lining. Common causes include:

• A stomach bug (gastroenteritis) — the classic trigger. An infection can strip the gut of lactase for a week or two, so dairy upsets you until the lining heals.
• Coeliac disease — undiagnosed coeliac damages the small intestine; the lactose intolerance often improves once a gluten-free diet lets the gut recover.
• Inflammatory bowel disease (Crohn's), gut surgery, or radiation therapy to the abdomen.

Is it permanent?

It depends on the cause. Post-gastro lactose intolerance is usually temporary, easing over roughly one to several weeks as the intestinal lining regenerates. If it's driven by an ongoing condition (like untreated coeliac or IBD), it tends to improve once that condition is treated. The primary, genetic kind is permanent — but even then, most people still tolerate some lactose. One honest note: while various medicines, surgery and radiation can injure the gut, we couldn't find good evidence that ordinary antibiotics directly cause lactose intolerance, so we won't claim they do.

What to do in the meantime

While your gut recovers, ease off high-lactose foods (our lactose content chart shows which to watch), lean on lower-lactose and lactose-free options, and reintroduce dairy gradually. If you want to confirm what's going on, see how lactose intolerance is tested, and what actually helps for managing it day to day.