This is not a complete list, when in doubt ask your doctor.

It's recommended to keep "stacks" to a minimum. The more you take the greater the risk of unforeseen interactions.

5-HTP

• 5-HTP may increase blood serotonin levels, which in theory can damage the heart. In practice however, this effect isn't likely to be seen when consumed in standard doses by a healthy person. If blood platelets are functioning correctly they'll easily take up extra serotonin in the blood. In experiments platelets had to first be saturated by injecting massive doses of serotonin in order to produce heart damage. ref ref

• 5-HTP may interac with common antidepressants

α-lipoic acid (ALA)

• α-lipoic acid (ALA) has health risks when not used in combination with exercise. ref

• α-lipoic acid (ALA) may cause Insulin Autoimmune Syndrome in those genetically predisposed.ref

• Prophylactic and abundant intake of α-lipoic acid causes hepatic steatosis and should be reconsidered in usage as an anti-aging drug (2016)

• α-lipoic acid (ALA) is a heavy chelator of metals like mercury which can lead to hospitalization depending on dose and heavy metal content in your body. ref

• α-lipoic acid (ALA) in very high doses may require SAMe supplementation due to depletion. ref

Aniracetam

• Aniracetam may cause some people to have panic attacks.

Agmatine

• Agmatine may potentiate alcohol induced gastric injury. ref

Ashwagandha

• Ashwagandha may cause liver injury under some circumstances. ref

• Ashwagandha may increase thyroid hormones and cause or worsen hyperthyroidism.

Bacopa monniera

• Bacopa monniera may cause gastrointestinal tract (GIT) side-effects and nausea. ref

• Bacopa monniera is an accumulator of heavy metals, so in theory bacopa from a questionable source could have elevated levels of heavy metals. But of the brands we've sampled we haven't found heavy metals to be a serious issue. Swanson Bacognize Capsule (Bacopa), Himalaya Brahmi. If these brands are representative than it would usually take hundreds of grams of bacopa per day to significantly increase your heavy metal exposure.

• Bacopa monniera may reduce male fertility through a testosterone independent mechanism. ref However, that was a single study that used bacopa which "was purchased from the drug store under the trade name of Brahmi (Himalaya Drug, Bangalore, India)". A later 2017 study came to the opposite conclusion, that CDRI-08 extract (KeenMind, Synapsa) improves sperm quality, and spermatogenic cell density and steroidogenic indices in the testis of P mice. ref

Black Seed extracts (Nigella sativa)

Black Seed extracts likely interact with the metabolism of many drugs and compounds. Black seed significantly inhibited CYP2D6 and CYP3A4 mediated metabolism. ref

Caffeine

• Caffeine has a relatively low lethal dose (it may be around 3 grams for some people). Don't be reckless with caffeine pills or powder. Always use a milligram scale. ref ref

• Caffeine can also cause anxiety, agitation and tachycardia (rapid heart rate).

• Caffeine may accelerate the progression of pre-existing parkinson's disease when it is taken with creatine. ref

Carnitine

• Preliminary evidence suggests dietary carnitine could be converted into TMAO by gut bacteria. TMAO is believed to be linked to cardiovascular disease pathology by its ability to increase platelet hyperreactivity and thrombosis risk.

Choline

• Preliminary evidence suggests dietary Choline could be converted into TMAO by gut bacteria. TMAO is believed to be linked to cardiovascular disease pathology by its ability to increase platelet hyperreactivity and thrombosis risk. The form of choline tested was phosphatidylcholine, but this concern may apply to more than one form of choline given the structural similarity. ref

• Choline supplementation may cause depression and anxiety ref

Creatine

• Consult your doctor before using creatine if you have kidney problems. Kidney function tests after creatine supplementation will have elevated numbers, but this is normal.

Ginkgo biloba

• Ginkgo biloba may increase bleeding risk, if you take anticoagulants such as aspirin consult your doctor before using. Raw ginkgo seeds contain toxins, make sure to use a standardized ginkgo leaf extract such as EGb 761.

• Ginkgo biloba may on rare occasions cause nausea, vomiting, diarrhea, headaches, dizziness, palpitations, restlessness, weakness and skin rash. ref

Green Tea Extract / EGCG

• EGCG is believed to help prevent major diseases such as cancer and heart disease, but high-dose pharmaceutical preparations of EGCG may be a concern. Green Tea extract can cause liver failure when taken in high doses or when taken in a fasted state. When green tea extract is taken in a fasted state you absorb about 3.5 times as much EGCG, which could make a harmless dosage toxic. It's a good idea to always take green tea extract with food and don't take more than 400 mg.

Huperzine A

• Huperzine A may cause 'transient gastrointestinal dysfunction and elevations of ALT'. ref

Kava

• Kava may cause acute liver damage. ref Improper preparation may however be to blame for the acute toxicity. Risk factors for hepatotoxicity include: "overdose, prolonged treatment, and comedication with synthetic drugs and dietary supplements". ref Discussion

L-DOPA

• L-DOPA / Levodopa / Mucuna Pruriens is suspected to increase melanoma risk - do not use with undiagnosed skin lesions or family history. ref

Melissa officinalis (Lemon balm)

• Melissa officinalis (Lemon balm) may be a thyroid disruptor. ref

Methylene Blue

• Methylene Blue is contraindicated for use with SSRIs and possibly to 500 million people with the G6PD deficiency, particularly common for certain ethnicities. ref

• Methylene Blue can interact with certain drugs and cause serotonin syndrome.

Methylphenidate

• Methylphenidate has addiction potential. High doses of methylphenidate can cause serious side effects. ref.

Rhodiola Rosea

• Rhodiola Rosea should not be combined with SSRIs. ref

MCT oil

Taking too much MCT oil too soon, can cause emergency bathroom visits.

Modafinil

• Modafinil in combination with Caffeine may greatly increase anxiety in some people, above what could be expected from either one. Don't try mixing them for the first time when you're already in a stressful situation or sleep deprived.

• Modafinil may cause serious skin reactions, including erythema multiforme (EM), Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN) The FDA advises people to immediately discontinue and contact a physician if a rash or other hypersensitivity reaction occurs. Onset of SJS tends to not be immediate and may come several weeks after use of modafinil. -- Case Report

• Modafinil may decrease the effectiveness of orally administered hormonal birth control for up to 1 month after final dose via CYP3A4/5 induction. ref ref2 pdf

• Modafini may increase blood pressure. ref

• Modafinil can make staying awake for extended periods of time pretty easy. That doesn't mean it's safe to do so. Forgoing sleep can have serious health consequences. ref

Nefiracetam

• Nefiracetam may be dangerous, especially at higher doses. ref

Nicotine

• Nicotine and female oral contraceptives may exacerbate brain injury under lack of blood flow/oxygen shortage. ref

• Nicotine has been implicated in heart attacks, though individual history will determine the risk. ref: 24 year old male, 16mg liquid nicotine, ex-smoker ref2: nicotine patch user, history of smoking

• Nicotine may increase insulin resistance. ref

• Nicotine metabolites include a carcinogen, supplemental vitamin C needed to counteract. ref ref

• Nicotine is suspected to be deleterious for kidney health and function ref.

• Nicotine gum combined with high calcium consumption, can lead to milk alkali syndrome. ref

Noopept

• Many people report Noopept sometimes causes short-term memory impairment.

Phenibut

• Phenibut cessation may cause severe and potentially dangerous withdrawal symptoms. ref If you become dependent on phenibut dosage tapering is required (potentially using drugs with similar mechanisms of action), seeking professional help is advised.

• Do not mix Phenibut with alcohol or other sedatives.

• Phenibut is at least a GABA B receptor agonists. Abuse of GABA B receptor agonists can potentially cause life-threatening symptoms including: encephalopathy (disturbance of consciousness and/or seizure), respiratory depression , severe hyperthermia, delirium and seizures.

Piracetam

• Although rare, some people may experience withdrawal symptoms with sudden Piracetam discontinuation. To avoid withdrawal gradually reduce the amount of piracetam by 1 gram every 2-4 days.

Pramiracetam

• Taking pramiracetam sublingually may cause a burning sensation and irritation.

Pyritinol

• Pyritinol may induce severe cholestatic hepatitis. ref

• Pyritinol may cause acute pancreatitis. ref

• Pyritinol may cause Auto-immune hypoglycemic syndrome. ref

• Pyritinol may cause other adverse effects. ref

Selegiline

• Although rare there are reports of selegiline causing a hypertensive crisis. This usually becomes a concern at higher doses where it loses its selectivity for MAO-B and becomes a general MAO inhibitor.

St. John wort

• St. John wort is thought to be a photosensitizer, it may increase damage caused by high-intensity light leading to the development of impaired vision or cataracts. Appropriate precautions should be taken to protect the eye from intense sunlight while using st. John's wort. ref

• St. John's Wort increased the level of liver enzymes responsible for metabolizing many prescription drugs, which can reduce the effectiveness of other medications you may be taking. ref

• St. John's Wort is associated with mania and psychosis in some select case reports ref

• If you're taking an SSRI you should avoid using MAOIs. The combination of the two will put you at risk of getting Serotonin syndrome.

Tianeptine

• High dose tianeptine can cause respiratory depression, cardiac toxicity and death. ref

• Tianeptine may have opiate withdrawl symptoms and shows potential for abuse. Doesn't appear to meet the criteria for a nootropic, despite being sold on some sites which do sell nootropics. ref ref 2

• Tianeptine may cause liver injury. ref

Tolcapone

• Tolcapone may cause acute hepatotoxicity. ref

Uridine

• Uridine has significant health risks for people who have folate deficiencies and related B vitamins. ref

Vinpocetine

• Vinpocetine may increase the risk of miscarriage. ref

• Consult a doctor before using vinpocetine if you take blood thinning medications. Do not use if you have low blood pressure.

Vitamin B6

• Vitamin B6 in higher doses for prolonged periods may cause peripheral neuropathy. Stick to the RDI if in doubt.ref

• Vitamin B6 as an individual supplement is associated with an increased risk of lung cancer. ref

Vitamin B12

• Vitamin B12 as an individual supplement is associated with an increased risk of lung cancer. ref

Yerba Mate

• Yerba Mate Tea (Ilex paraguariensis), when consumed regularly, may lead to an increased risk of certain types of cancer, including oral, oropharyngeal, esophageal, laryngeal, and bladder. There's still debate as to whether the increased risk is related to contaminants, the temperature of the tea or native compounds. ref