"You are what you eat." That well known phrase applies to vocal health. Foods and drink have effects on the vocal folds as well as the other components of the vocal tract. Dairy products are known to thicken secretions, and can affect voice quality accordingly. Spicy, fatty and acidic foods can increase reflux reaching the larynx. Fruits such as pineapple and papaya contain enzymes that can irritate tissues. Chocolate can cause vocal fold drying, as can coffee, tea and other caffeine containing drinks. Alcohol also tends to dehydrate the vocal folds. 

In general, moderation is the key to a voice healthy diet. Avoid large meals, particularly late at night. Be especially aware of what you ingest just prior to performance. Counteract drying effects with plenty of water intake. If you suspect allergies to certain foods, try eliminating these entirely from the diet.

General physical conditioning also plays an important role in voice health. A diet that promotes a lean body mass, a healthy heart, and good general stamina is beneficial to vocal support. Vitamin supplementation can aid in good nutritional support, but megadoses of vitamin C can result in more acid exposure for the larynx. Again, moderation seems prudent.

Many medications can have effects on the vocal tract. The drug table section covers these in detail.  Please consult with your doctor before making any changes in prescribed medications, even if you suspect an adverse voice effect.

Most often, it's wise to avoid over medicating during cold and allergy events. The medication effects can sometimes worsen the effects on the vocal folds. In particular, antihistamines and decongestants can be quite drying, and should be taken with caution and plenty of water to counteract  the effect. Menthol containing cough drops should probably be avoided, but glycerine based lozenges without a lot of additives can help sooth and lubricate inflamed tissues. If you have a cold or severe allergy flare up lasting more than eight days, you should seek medical care to avoid a more prolonged sinusitis or bronchitis event from developing