During the clinical trials described above, some people taking large amounts of 5-HTP experienced gastrointestinal upset (e.g. nausea) or, less often, headache, sleepiness, muscle pain, or anxiety.

A substance known as "Peak X" has been found in low concentrations in several over-the-counter 5-HTP preparations. Some researchers think this substance may be linked to toxicity previously reported in a 1989 L-tryptophan contamination incident. However, there is serious question about whether Peak X is actually the toxic agent and it may be unrelated to the problems previously associated with L-tryptophan. Although two articles reported possible associations between 5-HTP consumption and toxicity symptoms similar to those attributed to contaminated L-tryprophan, evidence linking 5-HTP or Peak X with any toxicity symptoms remains speculative. Although the structure of Peak X has recently been identified, there is no firm evidence that this substance has caused or contributed to any toxicity or disease.

Very high intakes of 5-HTP have caused muscle jerks in guinea pigs and both muscle jerks and diarrhea in mice. Injected 5-HTP has also caused kidney damage in rats. To date, these problems have not been reported in humans. "Serotonin syndrome," a serious but uncommon condition caused by excessive amounts of serotonin, has not been reported to result from supplementation with 5-HTP; in theory it could be triggered by the supplement. However, the level of intake at which this toxic effect might potentially occur remains unknown.

5-HTP should not be taken with antidepressants, weight-control drugs, other serotonin-modifying agents, or substances known to cause liver damage, because in these cases 5-HTP may have excessive effects. People with liver disease may not be able to regulate 5-HTP adequately and those suffering from autoimmune diseases such as scleroderma may be more sensitive than others, to 5-HTP. These people should not take 5-HTP without consulting a knowledgeable healthcare professional. The safety of taking 5-HTP during pregnancy and breast-feeding is not known at this time.

Avoid prolonged exposure to sunlight. 5-hydroxytryptophan may increase the sensitivity of your skin to the sun. Use a sunscreen and wear protective clothing when exposure to the sun is unavoidable.

In a controlled trial, 5-HTP (300 mg per day) was shown to be effective in reducing many symptoms of fibromyalgia, including pain, morning stiffness, sleep disturbances, and anxiety.

For depression, 300 mg per day is often effective, though much of the research used 5-HTP in combination with drugs or was uncontrolled.

For insomnia, a single 100-mg nighttime dose of 5-HTP was sufficient to improve the duration and depth of sleep in one placebo-controlled trial.

For migraine headaches, amounts ranging from 400–600 mg per day have been shown to be effective at reducing the frequency and severity of attacks in most clinical trials.

For tension headaches, 100 mg of 5-HTP taken three times per day led to a significant decrease in consumption of pain-relievers, but no significant change in headache duration or intensity.

Appetite reduction and weight loss (averaging 11 pounds in 12 weeks) has occurred with amounts of 600–900 mg daily. In another clinical trial, 750 mg per day has been shown to be effective at decreasing carbohydrate and fat intake, and promoting weight loss.

Certain medicines interact with 5-hydroxytryptophan: Some interactions may increase the need for 5-hydroxytryptophan (Beneficial), other interactions may be negative (Avoid) and indicate 5-hydroxytryptophan should not be taken without first speaking with your physician or pharmacist, others may require further explanation (Check).

• Carbidopa - Check
• Carbidopa/Levodopa - Check
• Fluoxetine - Avoid
• Fluvoxamine - Avoid
• Paroxetine - Avoid
• Sertraline - Avoid
• Sibutramine - Avoid
• Sumatriptan - Avoid
• Tramadol - Avoid
• Venlafaxine - Avoid
• Zolmitriptan - Avoid
• Zolpidem - Avoid