Alli is generally considered safe for many adults, but it can cause side effects, especially if you eat high-fat meals while taking it.

Common Side Effects

Because Alli blocks fat absorption, most side effects involve the digestive system:

• Oily or fatty stools
• Loose stools or diarrhea
• Gas with oily discharge
• Frequent bowel movements
• Sudden urge to have a bowel movement
• Stomach discomfort or cramps
• Oily spotting on underwear

These effects are usually more noticeable when you consume meals high in fat.

Effects on Vitamins

Alli can reduce the absorption of fat-soluble vitamins:

• Vitamin A
• Vitamin D
• Vitamin E
• Vitamin K

For this reason, many healthcare providers recommend taking a multivitamin at least 2 hours before or after Alli, often at bedtime.

Less Common Side Effects

• Headache
• Back pain
• Mild stomach pain
• Changes in bowel habits
• Fatigue

Rare but Serious Risks

Liver Problems

Rare cases of severe liver injury have been reported.

Seek medical attention if you develop:

• Yellowing of the skin or eyes
• Dark urine
• Severe fatigue
• Persistent abdominal pain

Kidney Stones

In some people, Alli may increase the risk of kidney stones, particularly those with existing kidney problems.

Allergic Reactions

Rarely:

• Rash
• Itching
• Swelling of the face, lips, or throat
• Difficulty breathing

These require immediate medical attention.

Who Should Avoid Alli?

Alli should generally not be used by people with:

• Chronic malabsorption syndrome
• Certain gallbladder disorders
• Known allergy to orlistat
• Pregnancy (weight-loss medications are not recommended during pregnancy)

Is Alli Dangerous?

For most healthy adults, Alli’s side effects are mainly digestive and are not dangerous. Serious complications are uncommon. Many people find that following a lower-fat diet significantly reduces the unpleasant gastrointestinal side effects.

Comparison With Other Weight-Loss Medications

Alli tends to have fewer systemic side effects than many prescription weight-loss drugs because it works primarily in the intestines rather than affecting the brain, appetite hormones, or metabolism.