Composition, what substances or chemicals does it have?
Celery seed has several sesquiterpenoid glucosides (celerioside A-E), phthalide glycosides (celephtalide A-C),  aromatic compound glucosides, norcarotenoid glucosides, and a lignan glucoside.

Benefit of Celery Seed
Very little research is available on celery seed. Celery seed extract may perhaps be helpful in liver cancer. In folklore, celery seeds have aphrodisiac properties.

Progress in Drug Research. 2015 .Celery Seed and Related Extracts with Antiarthritic, Antiulcer, and Antimicrobial Activities. Celery preparations have been used extensively for several millennia as natural therapies for acute and chronic painful or inflammatory conditions. This article reviews some of the biological and chemical properties of various celery preparations that have been used as natural remedies. Celery Seed Extract (CSE) and has been found to be at least as effective as aspirin, ibuprofen, and naproxen in suppressing arthritis in a model of polyarthritis. CSE can also reduce existing inflammation in rats. CSE may be a prototype of a natural product that can be used therapeutically to treat arthritis and other inflammatory diseases.

Celery seed side effects, adverse reactions, interactions with medications and herbal supplements
Q. I have come about Australian incident report on celery seeds and possible interference with thyroxine medication. I do research to bring about the best supplement for equine. I already get your emails, and in my search came up on your page on celery. This might help you as it can be an interesting observation. In the horse world I came upon that celery seeds has been associated with inflaming the kidneys if used long term. The medical article I came across says, "Case reports: Our first case involved a 55-year-old woman who, after considerable monitoring, had finally been stabilised on a daily dose of thyroxine 100 microgram. A month later, her doctor found that her T4 levels were low again and her dose was doubled. The patient then remembered that in the past month she had also started taking celery seed tablets for osteoarthritis. Suspecting a potential interaction, she ceased the celery seed tablets without increasing the thyroxine dose as the doctor had advised. Next time her thyroxine levels were checked they had increased to within the normal range. She tried recommencing celery seed a month later but after a week she felt lethargic, bloated and had dry skin. When she stopped the celery seed tablets, she reported that her 'general energy levels improved'.
   A second report was received from a 49-year-old woman who had taken thyroxine for many years. When her T4 became extremely low her doctor suspected that she had not been taking her tablets. The patient argued that she had taken her thyroxine, but she had recently commenced taking celery seed tablets to treat arthritis. She ceased the celery seed tablets and one month later her thyroxine levels had returned to within the normal range." Aust Prescr 2001.
   A. This is interesting. I had not seen this article before regarding the side effects of celery seed extract supplements.

Postgrad Med. 2016. Celery root extract as an inducer of mania induction in a patient on venlafaxine and St John's Wort. Celery root belongs to a group of plants which contains phytoestrogens, structurally similar to estrogen as they share a pair of hydroxyl groups and phenolic ring, which enables them to bind to estrogen receptors directly, making them a herbal remedy for low estrogen states such as menopause. We present a case of a female patient with depression who was stabilized on venlafaxine and St John's Wort, and who developed a manic episode due to elevated serum venlafaxine levels after she started taking celery extracts for menopausal related issues.

Research
Inhibitory effect of celery seeds extract on chemically induced hepatocarcinogenesis: modulation of cell proliferation, metabolism and altered hepatic foci development.
Cancer Lett. 2005.
The chemopreventive activity of methanolic extract of Apium graveolens seeds (celery seeds) has been investigated against Solt Farber protocol of hepatocarcinogenesis, oxidative stress and induction of positive foci of gamma-GT in the liver of Wistar rats. The prophylactic treatment of celery seeds extract protected dose dependently against diethylnitrosoamine (DEN)+2-acetylaminofluorine (AAF)+partial hepatectomy (PH) induced hepatocarcinogenesis. On the basis of the above results it can be said that celery seed is a potent plant against experimentally induced hepatocarcinogenesis in Wistar rats.

Potential of crude seed extract of celery, Apium graveolens, against the mosquito Aedes aegypti (Diptera: Culicidae).
J Vector Ecol. 2004.
Crude seed extract of celery, Apium graveolens, was investigated for anti-mosquito potential, including larvicidal, adulticidal, and repellent activities against Aedes aegypti, the vector of dengue haemorrhagic fever. The ethanol-extracted A. graveolens possessed larvicidal activity against fourth instar larvae of Ae. aegypti with LD50 and LD95 values of 81 and 176 mg/L, respectively. The abnormal movement observed in treated larvae indicated that the toxic effect of celery seed extract was probably on the nervous system. In testing for adulticidal activity, this plant extract exhibited a slightly adulticidal potency. It showed repellency against Ae. aegypti adult females with ED50 and ED95 values of 2 and 28 mg/cm2, respectively. It also provided biting protection time of 3 h when applied at a concentration of 25 g%. Topical application of the ethanol-extracted celery seed extract did not induce dermal irritation. No adverse effects on the skin or other parts of the body of human volunteers were observed during 3 mo of the study period or in the following 3 mo, after which time observations ceased. Celery seed extract, therefore, can be considered as a probable source of some biologically active compounds used in the development of mosquito control agents, particularly repellent products.

Celery stem and leaves
Jundishapur J Nat Pharm Prod. 2014. The Effects of Hydroalcoholic Extract of Apium graveolens Leaf on the Number of Sexual Cells and Testicular Structure in Rat. Use of medicinal plants with high antioxidant properties could be effective to increase fertility and improvement of disorders such as hormonal imbalance, impotency, oligospermia and immotile sperm. Celery (Apium graveolens) is rich in antioxidant agents. The leaf and stems of celery contain phenols, furanocoumarin and luteolin. Apigenin is one of the main flavonoids of celery leaf. It seems that celery increases spermatogenesis in male rats, but has no destructive effects on testicular tissue.

Emails
Q. I get a relaxed feeling when I eat a lot of celery.

Q. I have seen articles indicating that celery seed extract and celery are beneficial for lowering high blood pressure. I don't believe I have seen celery seed on your website. Do you have any information on it and would you be carrying a celery seed supplement in the future?
   A. We have not looked into celery seed in any detail yet, but we will look for more research in the future.

I have been taking cellery seed for about 10 years for gout. I take 1,500 mg both AM and PM. Recently I started taking Doxepin for depression. Soon after I started
having heartburn problems. I never had any symptoms of heartburn -  I am 56 year old man in generally excellent health. At first I though the heartburn was due to lactose intollerance. However, after slow and careful modification to what meds and food I was eating it is clear to me that the cause is the interaction of the cellery seed with the Doxepin. I take 225 mg of doxepin at the present time. I am wondering if you have ever heard of any one else with a similar issue with drug interaction with cellery seed. Currently I am back on allopurinol and seem to be symptom free although it has only been a short time. Any information you might have on this would be appreciated. It might also be good to post this on your web site.
    I have not had any feedback from anyone else thus far regarding the interaction of celery seed extract and prescription medications.
        I may have pulled the trigger too soon - O stopped the celery seed pills and still having issues with heart burn.