Excessive amounts may raise blood pressure, cause fluid retention and should be avoided during pregnancy and by anyone with heart or kidney ailments. Prolonged use may cause hypertension, edema, headache, vertigo and potassium depletion, also sodium and fluid retention.¹ Do not use if you have a history of stroke or if on digoxin. Products that contain licorice as a flavoring, such as chewing tobacco, have also been implicated in cases of toxicity. Hypersensitivity reactions to glycyrrhiza containing products have also been noted.

Actions: Accelerate the healing of gastric ulcers. Secretolytic and expectorant effects. Antispasmodic action.

Use: For catarrhs of the upper respiratory tract and gastric/duodenal ulcers.

Contraindications: Cholestatic liver disorders. Liver cirrhosis. Hypertonia. Hypokalemia. Severe kidney insufficiency. Pregnancy.

Side Effects: With prolonged use and higher doses, there may be sodium and water retention and potassium loss, accompanied by hypertension, edema, and hypokalemia, and myoglobinuria (rare).

Cautions: Other drugs (such as thiazide diuretics) may increase potassium loss. Sensitivity to digitalis glycosides increases with the decrease in potassium.

Duration of Administration: Do not use longer than 4 - 6 weeks without medical advice. There is no objection to using licorice root as a flavoring agent up to a maximum daily dosage equivalent to 100 mg glycyrrhizin.
Source for the above reference: http://neuro.vetmed.ufl.edu/Alt_med/Library/Monograph/licorice.html

¹ Sigurjonsdottir HA, et al, J Human Hyper ten  1995;9(5):345-348

More Reference Material:

1. Cook, M. K. and Gorminger, B. H., International Symposium on Sweeters (ed. Inglett, G. E.), Avia. Publ., Westport, Conn., 1974, pp. 19–24.
2. Beil W, Birkholz C, Sewing KF. Effects of flavonoids on parietal cell acid secretion, gastric mucosal prostaglandin production and Heliobacter pylori growth. Arzneim Forsch 1995;45:697-700.
3. Turpie AGG, Runcie J, Thomson TJ. Clinical trials of deglycyrrhizinated liquorice in gastric ulcer. Gut 1969;10:299-302.
4. Snow J. Monograph: Glycirrhiza glabra Leguminacae The Protocol Journal of Botanical Medicine 1996;Winter:9-14.
5. van Marle J, Aarsen PN, Lind A, et al. Deglycyrrhizinised liquorice (DGL) and the renewal of rat stomach epithelium. Eur J Pharmacol 1981;72:219-25
6. Morgan AG, McAdam WA, Pacsoo C, et al. Comparison between cimetidine and caved-s in the treatment of gastric ulceration, and subsequent maintenance therapy. Gut 1982;23:545-51.
7. Stormer FC, Reistad R, Alexander J. Glycyrrhizic acid in liquorice- evaluation of a health hazard. Fd Chem Toxic 1993;31:303-12.
8. Murray, MT: Encyclopedia of Nutritional Supplements. Prima Publishing, Rocklin, CA; 1996.
9. Das SK, Gulati AK & Singh, VP: Deglycyrrhizinated Liquorice in Apthous Ulcers. J Assoc Physicians India 1989;37:647.
10. Morgan AG et al: Comparison between Cimetidine and Caved-S in the treatment of gastric ulceration and subsequent maintenance therapy. Gut 1982;23:545-551.
11. Kassir, AZ: Endoscopic controlled trial of four drug regimens in the treatment of chronic duodenal ulceration. Irish Med J 1985;78:153-156.
12. Turpie AG, Runicie J& Thomson TJ: Clinical trial of Deglycyrrhizinated liquorice in gastric ulcer. Gut 1969;10:299-303.
13. Rees WDW et al: effect of Deglycyrrhizinated liquorice on gastric mucosal damage by aspirin. Scan J Gastroent 1979;14:605-607.
14. Tewari SN & Wilson AK; Deglycyrrhizinated liquorice in duodenal ulcer. Practitioner 1972;210:820-825.
15. M. Paolini et all. Effects of liquoirce and glycyrrhizin on rat liver carcinogen metabloizing enzymens http://147.46.94.112/journal/sej/full/c02_199916_1450105.pdf