Side effects are rare but you may experience some, such as nausea (a feeling of sickness), drowsiness, headache and light-headedness. These tend to wear off after a few days. If they do not, or cause you discomfort, you should discuss this with your doctor. It may be possible to adjust your dose to reduce these effects. Taking your tryptophan after food can help to prevent it from making you feel sick. It is not thought that the drug has any major long term side-effects.

In the 1990s there were some serious and unpleasant side-effects with health food products in America which contained tryptophan. These side effects were caused by contaminants introduced due to poor processing.  The industry has learned from those mistakes and now takes extra precautions to avoid such contamination from occurring presently and in the future.  Tryptophan itself was not directly linked with these side-effects. If you suffer any severe joint or muscle pain, skin rash, swelling or fever please tell your doctor immediately and never take tryptophan that was packaged in the 1990s and never take tryptophan that is not labeled with a production date.

Tryptophan may make you feel drowsy. You should not drive or operate machinery until you know how it affects you. You should be careful as it may affect your reaction times. Tryptophan will not however make you fall asleep like some sleep aids, although if you take it at night it may help you get to sleep faster.

Contrary to popular belief, tryptophan in turkey meat does not cause drowsiness. Turkey does contain tryptophan, which does have a documented sleep-inducing effect. However, tryptophan is only effective when taken on its own as a free amino acid. Tryptophan in turkey is found as part of a protein, and in small enough amounts that this mechanism seems unlikely.

A more likely hypothesis is that the ingestion of large quantities of food, such as at a Thanksgiving feast, tends to induce drowsiness. Whether this drowsiness is related to elevated hypothalamic serotonin levels following feeding or is mediated by some other mechanism is unknown.