Before taking any medication, always read the package instructions and ask your doctor or pharmacist about any dietary precautions. In some cases, drugs alter nutritional needs; in other instances, foods can interfere with how a medication works. The table below details how particular foods can interact with some of the more commonly used drugs/medications.

Drugs  Effects and Precautions    

ANTIOBOTICS

Cephalosporins, penicillin: Take on an empty stomach to speed absorption of the drugs.
Ciprofloxacin: Avoid dairy products, caffeine, and supplements, which contain calcium, iron, or zinc, for 2 hours before and after taking the medication.  
Erythromycin: Don't take with fruit juice or wine, which decreases the drug's effectiveness.  
Sulfa drugs: Increase the risk of vitamin B12 deficiency.
Tetracycline: Dairy products decrease the drug's efficacy. Lowers vitamin C absorption.

ANTICOAGULANTS

Dilantin, phenobarbital: increase the risk of anemia and nerve problems due to a deficiency of folate and other B vitamins.

ANTIDEPRESSANTS

Fluoxetine: Reduces appetite and can lead to excessive weight loss.
Lithium: A low-salt diet increases the risk of lithium toxicity; excessive salt reduces drug's efficacy.
MAO inhibitors: Foods high in tyramine (aged cheeses, processed meats, legumes, wine, beer, among others) can bring on a hypertensive crisis.
Tricyclics: Many foods, especially legumes, meats, fish, and foods high in vitamin C, reduce absorption of the drugs.

ANTIHYPERTENSIVES, HEART MEDICATIONS

ACE inhibitors: Take on an empty stomach to improve the absorption of the drugs.
Alpha blockers: Take with liquid or food to avoid an excessive drop in blood pressure.
Antiarrhythmic drugs: Avoid caffeine, which increases the risk of an irregular heartbeat.
Beta blockers: Take on an empty stomach; food, especially meat, increases the drugs' effects and can cause dizziness and low blood pressure.  
Digitalis: Avoid taking with milk and high-fiber foods, which reduce absorption. Increases potassium loss.
Diuretics: increase the risk of potassium deficiency.
Potassium-sparing diuretics: Unless a doctor advises otherwise, don't take diuretics with potassium supplements or salt substitutes, which can cause potassium overload.
Thiazide diuretics: increase the reaction of MSG. 

ASTHMA DRUGS

Pseudo-ephedrine: Avoid caffeine, which increases feelings of anxiety and nervousness.
Theophylline: Charbroiled foods and a high-protein diet reduce absorption. Caffeine increases the risk of drug toxicity.

CHOLESTEROL-LOWERING DRUGS

Cholestramine: Increases the excertion of folate and vitamins A,D,E, and K.
Gemfibrozil: Avoid fatty foods, which decrease the drug's efficacy in lowering cholesterol. 

HEARTBURN AND ULCER MEDICATIONS

Antacids: Interfere with the absorption of many minerals; for maximum benefit, take medication 1 hour after eating.
Cimetidine, famotidine, sucralfate: Avoid high-protein foods, caffeine, and other items that increase stomach acidity.

HORMONE PREPARATIONS

Oral contraceptives: Salty foods increase fluid retention, Drugs reduce the absorption of folate, vitamin B6, and other nutrients; increase intake of foods high in these nutrients to avoid deficiencies. 
Steroids: Salty foods increase fluid retention. Increase intake of foods high in calcium, vitamin K, potassium, and protein to avoid deficiencies.
Thyroid drugs: Iodine-rich foods lower the drugs' efficacy.

LAXATIVES

Mineral oils: Overuse can cause a deficiency of Vitamin A, D,E, and K.

PAINKILLERS

Aspirin and stronger non-steroidal anti-inflammatory drugs: Always take with food to lower the risk of gastrointestinal irritation; avoid taking with alcohol, which increases the risk of bleeding. Frequent use of these drugs lowers the absorption of folate and vitamin C.
Codeine: Increase fiber and water intake to avoid constipation.

SLEEPING PILLS, TRANQUILIZERS

Benzodiazepines: Never take with alcohol. Caffeine increases anxiety and reduces the drugs' efficacy.